Between my a1200 and amithlon box Im perfectly content as far as my classic amiga interest goes. Amithlon provides more hardware grunt than any of the ppc options (even running 68k code vs ppc code, let alone running x86 native amiga os software), which lets you do things like watch movies, watch t.v (via tv tuner card and amithlontv)play quake1/2, freespace, etc. at high framerates (over 100fps in q2 using software rendering in n640x480 for example), etc. and without the need to install windows, linux, etc. Simply install amithlon and os3.9, and the pc is suddenly an RTG/RTA based "amiga"

It's amazing how capable os3.x is still with some customisation and some hardware grunt behind it. I guess some of this applies to x86 based UAE as well, but the experience is so much more pleasant with amithlon for my tastes (not just cos of being faster). It's also a lot more authentic an experience. A uae user for example doesnt need to worry about file systems, tcp/ip stack, cant create custom rtg screenmodes and a variety of other "amiga" things. Amithlon behaves exactly like an amiga.
MOS is undoubtedly a little nicer than a tweaked os3.9 system (even if having lesser hardware resources than amithlon on a modernish machine), but for me there's a charm in the more or less D.I.Y nature of os3.x, and it's a lot of fun building up. That said though I do like MOS, theyre just different aspects of the same hobby.
Give it a try if you get a chance, you might enjoy it
