My boyfriend built me a cat tree, and we covered it in sisal and carpet. I also have one store bought tree that has some of that softer fluffy stuff at the top. I can honestly say, the carpet and sisal are where it is at for climbing and really digging claws into things. The one that has fluff is great for sleeping, but he doesn't use it for anything else. He really tries to tear up the carpet and sisal on the other trees though, store bought or home built.
Some thoughts I have on cat trees: the tree should be high enough, with one continuous length for the cat to stretch out completely for clawing. That is, for a cat that could stretch out to be 3ft (arms above head), the tree should have some sort of vertical scratch friendly material (carpet or sisal). This is something you can see outdoor cats do on actual trees, and when I made Midas indoors only, something I really wanted him to still be able to do in some capacity. So personally, I do not think that any tree under 2ft is appropriate as the sole scratching post for an indoor only cat unless other scratchers are provided (like cardboard). Second is my preference for sleeping spots to be large enough for the cat to stretch out on while sleeping. I don't think this is as important as the scratching ability, but if possible it should be considered.