Hard question to answer. Yes... it is very true that most Windows Servers degrade performance wise over time even if not updated. For example, file system fragmentation.

Unix/Linux hosts will have much less fragmentation issues file system wise.

However, both Unix (AIX) and Linux can degrade as updates are made, especially if additional services are brought online, extra security is inserted or just growth of various programs. But in general, those items are not "big" killers of performance.

So... if vs. Windows, you can bet that the majority of it is OS related.

Linux is a good choice and much more affordable than commercial Unix (e.g. AIX) for learning some enterprise level software. Most of the major enterprise software manufacturers have excellent support Linux now.

With that said, not all hardware is created equal. "Cheap" consumer hardware is typically where the most problems are. You generally don't want bleeding edge because Linux might not support everything there. Usually big vendors have compatibility lists which will tell you whether or not Linux is supported on their servers (e.g. Dell, HP and IBM). Those same "big vendors" may also have consumer line hardware which is *not* ok for Linux... so do your research wisely.