I'm a die-hard Mac user. I have four Macs here, no PCs. MacMAME works great for me, and SDL-MAME works fine in my G5 system. I love my Macs. . . .
Having said that, I'm going for a refurb Windows PC as the heart of my MAME cabinet. There are just too many utility programs that are only for Windows. That includes front ends, it includes ROM management tools, and also configuration tools for the U360 sticks.
I'm going to try Win98 SE first and see how that goes.
I'm not interested in running anything besides MAME on mine. No web surfing, no jukebox, etc. I have my Macs for all that stuff.