If it's big enough, no difference - man would be wiped out.

I would guess that at an angle would be worse.

Regardless of the angle, the crater would be a perfect circle, something you can demonstrate for yourself by filling a large pan full of flour and tossing marbles into it from various angles. Hitting at an angle might affect the seasons, though. It all depends on the size and speed.

The best clues are to be found in point 5. For a low angle, you get a smaller crater and a weaker shock wave. So a low angle would be better, I think. (You could also get a ricochet or an explosion in the atmosphere before impact, with a more widespread damage. But I was assuming one single impact) 1) "Impact cratering involves collisions between solid objects at high speeds; typically the velocity of impact is higher than the velocity of sound in those objects. Such hyper-velocity impacts produce physical effects, including melting and vaporization, that are quite different from those that occur in familiar sub-sonic collisions. On Earth, ignoring the effects of travel through the atmosphere, the lowest velocity at which impact on the surface can occur is the gravitational escape velocity of about 11 km/s. The fastest impacts occur at more than 70 km/s which represents the sum of the escape velocity from Earth, the escape velocity from the Sun at the Earth's orbit, and the motion of the Earth around the Sun. The median impact velocity on Earth is in the region 20 to 25 km/s. Impacts at these high speeds produce shockwaves in solid materials, and both the impactor and the material impacted, are rapidly compressed to high density. Following this initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of the material in most impacts is many times higher than that in the highest high explosives. Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters." Source and further information: http://en.wikipedia.org/wiki/Impact_crater 2) "Whether an object breaks apart depends on its composition, speed, and angle of entry. A faster meteor at an oblique angle suffers greater stress. Meteors made of iron withstand the stress better than those of stone. Even an iron meteor will usually break up as the atmosphere becomes more dense, around 5 to 7 miles up. A meteor sometimes explodes above the surface, causing widespread damage from the blast and ensuing fire." Source and further information: http://www.crystalinks.com/meteors.html 3) "Melosh (1989), citing hypervelocity experiments, notes that only highly oblique impacts (angle less than 10 degrees) create asymmetrical impact craters. These asymmetrical impacts make craters with a rim located on the same side as the approach trajectory. Such rim development is due to the fate of the impactor because it disintegrates and is jetted horizontally uprange (in the direction of travel; Melosh, 1989, see his Figure 4.3)." "Global and local atmospheric effects of Wetumpka or any impact event large enough to register a crater equal to or exceeding Wetumpka's may include: (1) cooling and photosynthetic suppression due to atmospheric dust loading, wafted soot from large-scale fires, shock pressure-generated nitrous-oxide, and target-generated SO2; (2) large-scale fires and associated atmospheric injection of pyrotoxins; (3) acid rain from pollution by burning, nitrous-oxide generation, and SO2 injections; (4) ozone loss due to nitrous- oxide generation; (5) mechanical pressure effects due to shock waves; (6) destruction and drowning due to tsunami; (7) global warming due to H2O and CO2 injections; (8) and water, food, and soil poisoning due to heavy-metal dispersion (Toon et al., 1994)." Source: http://www.auburn.edu/~kingdat/wetumpkawebpage3.htm 3) The speed will be the same if the relative speed of the asteroid and of earth are the same. In most cases, we can ignore the rotational speed of both on themselves and only consider the linear speed (speed of their center of mass). I also assume that there will be no bouncing off or no exploding inside the atmosphere before the impact. Bouncing off would probably create less damage and exploding before impact more, extensive damage. Anyway, I think that in the case of a very low angle, the damage will be spread on a larger region and that it will be a greater damage. 4) The most interesting for us is the damage caused by the asteroid on the surface (superficy) of the Earth. If the asteroid caused an important, not only superficial damage, it would not make much difference for us, because all higher life would probably be erased... Incidentally, by an oblique impact, they could be a modification of the Earth's rotation speed or even of the Earth's rotation axis. Further information: http://en.wikipedia.org/wiki/Asteroid_impact 5) "Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters’ rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle" Source: http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.28.1.141 6) Interesting article about the ejection from Mars material into space by an impact on the martian surface: "A high-velocity oblique impact into the martian surface accelerates solid target material to escape velocity. A fraction of that material eventually falls as meteorites on Earth. For a long time they were called the SNC meteorites (Shergotty, Nakhla, and Chassigny). We study production of potential martian meteorites numerically within the frame of 3D hydrodynamic modeling. The ratio of the volume of escaping solid ejecta to projectile volume depends on the impact angle, impact velocity and the volatile content in the projectile and in the target. The size distribution of ejected fragments appears to be of crucial importance for the atmosphere–ejecta interaction in the case of a relatively small impact (with final crater size <3 km): 10-cm-sized particles are decelerated efficiently, while 30–50% of larger fragments could escape Mars." Source: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4CMYK48-2&_user=10&_coverDate=09%2F30%2F2004&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=32c2cd9bc49eca43e72992f9b691a00e

If it's really big, it won't matter how it hits, it will still do massive damage. However, if there was one, say, the size of a Volkswagen Bug, it would be worse to come straight down. If the asteroid comes in the atomospher at an angle, it will have more time to break apart and burn up before it hits, making a less destructive impact. But of course, an asteroid can't hit earth from straight up and down, because earth's movement around the sun and our gravitational field won't allow it.