Well, do you want to use wireless? If the two computers are near each other, wired is fine. If you can't conveniently run a cable to each computer, wireless may be your best choice. Obviously, a wireless router will cost more, as will wireless cards for the PCs. It's also somewhat slower than a wired network, which isn't a problem for sharing Internet, but will be a slowdown for sharing large files.
Wireless provides more flexibility if you intend to move the computers, or buy a laptop. Keep in mind that wireless routers will include wired points as well, so if you buy a wireless router you can use a combination of wireless and wired devices.