I don't think boot speeds are too much of an issue these days. Not only do you have the OS boot delay, but the bios of your motherboard will influence the boot time. Everyone seems to accept the boot time as part and parcel of a MAME cab, so whatever you choose just do all the bios/OS software mods you can to get to your frontend that little bit faster if it bothers you. If seemless booting is an issue, be sure to set your front-end as the boot shell and turn off any bios displays if you can.
I opt for XP due to it's compromise of ease of use and tuneability, as those RAM-sapping services can all be turned off anyway using regedit, plus USB updating is so much easier due to theplug and play drivers which 98 doesn't have. You can also wipe the boot screens easily or swap them for MAME-based alternatives.