Copper

Depending on the wattage and ohms more than likely you will want to run stranded copper 16 AWG wiring. Be sure to use the in-wall type of wiring with the plastic jacket covering the two conductors.

That's a heck of a long run to wire and needs to be done with care to avoid problems with the sound or the amplifier. You want to ensure that the electrical resistance of the wire is kept to a minimum. The length of both speaker leads needs to be taken into consideration and the total resistance of the 150 ft run (2x75) should be kept under 1 ohm, equivalent to 0.007 ohms/ft. A resistance of 0.002 ohms/ft would produce a resistance of 0.3 ohms, which is reasonable. The capacitance of the wire should also be considered and it is also something you need to minimize. Since capacitance is generated between the leads, you only need to consider the 75 ft run. Capacitance varies widely among different wires and you need to keep it as low as possible. High capacitance may cause problems in the output stage of an amplifier. Most wire manufacturers provide technical information on-line, particularly large companies like Belden. If you are looking to a wire marketed specifically as a speaker wire, some of these manufacturers provide information line. Kimber Kable makes quite good speaker cables and some of them are quite modestly priced. Kimber 8PR would be quite suitable, the less expensive 4PR might also have suitable characteristics. The ubiquitous Monster Cable sells a wide range of products. They are somewhat overpriced for the quality you receive - all their promotion for bargain and mid-range audio dealers costs money - but their products widely available. Any wire run out of doors needs to be placed inside a metal or plastic conduit for electrical safety purposes and to prevent damage to the wire. Metal conduit is preferred and should be grounded to earth. Once again I recommend that you always check the wire specs. 150 ft of wire is going to cost you well over $150 if you buy decent wire. Even good-quality 14 or 16 gauge extension cord wire, something I cannot recommend for use as speaker wire, will cost over $0.50/ft.