All other things being equal, Oracle has ALWAYS worked best on SUN.
Another thing to consider is that Oracle (corp) says “Oracle (the db) is Oracle is Oracle” meaning that if you say “ABC…XYZ” within Oracle it will result in the same answer regardless of the hardware or OS. PART of the reason for this is that the Oracle database engine is generic. Even on a mainframe running z/OS (back when this was supported) any given Oracle command or statement would work the same as it did on a PC. (obviously there were other layers added between Oracle and the OS, but that is another topic.)
The point of the previous paragraph is that you need to consider your OS knowledge when choosing hardware or OS.
Since I am a bigot, might I suggest that you consider running DB2 on a “real” computer (mainframe)? 🙂