The square are the way to go if you can afford them. They simply have better specs for a given size, but they cost much more. Start searching ebay for THK, Thomson, IKO, INA, Linear, bearings, linear actuators, CNC, etc. Do your homework now, check out all the DIY builds here, this site is a fantastic wealth of info. First of all define your requirements. If you need to cut heavy aluminum your needs are much different than if your just engraving into wood, of course. If you use a bidnapper service you can pick up linear square rails off ebay for a song. Spend a little time planning and deciding what you really need first, that way you wont be starting your second machine before your first one is done, like I did! Good luck and start a thread when you get started.
Halfnutz
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)