Buying a tree is a little bit of a crap-shoot in my experience. I have some criteria like:
* tree should be 6' or higher
* should have at least one perch (per cat) with a nice wide base that can accommodate a cat bed on it
* wide base (for stability)
* sisel rope near ground level for scratching
* ideally has some fairly long poles for climbing
* there needs to have a way down from the high perches without a "crash landing" onto the ground (very important! - the first thing I do when I see a tree is try to trace such a path down with my eyes without having to jump 6' down to the floor).
* hanging toys from a springy string are a HUGE plus.
Aside from that though, one big factor is how well the whole thing fits together. I ended up throwing one tree into the trash after buying it, even though it had all my criteria, simply because it fit together so poorly and ended up being a rickety mess.
One really good alternative (or can even work in conjunction with) a cat tree are perches on the wall, or "hi cat" poles.
This stuff is ridiculously high quality:
http://www.catsplay.com/cat-wall-climbing-systemsAnd this is really neat too:
http://www.hicat.co.uk/Hope that helps.