id go for the biggest, sturdiest cat tree you can afford. and, one that screws into the wall.... mine like ones where they can fully stretch out and that is flat - so no shelves hitting them in the tummy when they are scratching.
i have that oasis zooplus one, and sometimes they go into the cave... but they dont really bother with it that much. It isnt big enough.
I also have a ancol fat boy, (
http://www.amazon.co.uk/Ancol-Acticat-Scratch-Post-70cm/dp/B001159A7W )which is actually quite sturdy - find it sturdier than the ultimate scratcher (
http://www.amazon.co.uk/Smart-Cat-Ultimate-Scratching-Post/dp/B000634MH8) they def use that the most for scratching.
the ancol doesnt screw into the wall, but it is big and my big boy can almost make it to the top of it. He's also been known to sit on it...
you can see it in this picture :
they also really use one i got from lidl/aldi a few years ago - it is screwed in which really helps with the stability. never touched the hammock though
what i have discovered, is that they dont tend to use the big one for scratching as they dont like going over the jutting out bits to scratch. find it hard to scratch over them. so they tend to use the really long bit only. (pic is old!) so when they want a scratch they go to the fatboy.