The ThermalTake XP120 is a very good performer, but it's design is also it's down fall. Like it was mentioned above, it simply doesn't fit on some MOBOS. Here is a site that lists it's compatibilities.
The xp90c is new and I haven't heard very much about it. It does look a bit smaller in design and it is all copper. All Copper is a good thing. Can't say much about it. ThermalTake is a very good company though, across the board.
My choice would be a "Zalman (all copper) 7700" or "Zalman 7000" it depends on your wallet.
I have the Zalman (all copper) 7700, it keeps my FX-55 extremely cool. USE Artic Silver 5 only!!!