Personally Athlon is my choice, cheaper and just as good. If you run linux alot then go for the dual CPU.
Palomino Athlon 1600+ is the best CPU I've ever owned. Stable, and the CPU die is huge, which makes it pretty easy to cool. Thouroughbred Athlons are a real pain. If I knew how hard it was to cool those tiny chips, I would've bought a P4. I am so sick of the noise, and watercooling seems like such a huge investment when I can just buy a P4 like my dad. His computer is SILENT, man. His monitor makes more noise than the computer!
As for MorphOS vs AmigaOS, I could care less. Use what works. Both are going to branch off into directions less and less "Amiga" (which might be a good thing). I don't know much about MorphOS, though, and I don't really care.
