my first rr install was on XPembedded, very stripped down, onto a high speed 2gb compact flash. my boot time was great - less than 10 seconds - however setting up roadrunner and all the applications i needed on XPe was very, very time consuming. and any time i needed to update something, i had to rebuild XPe because of how it worked. i now use a solid state drive and hibernation now anyway, so boot time no longer concerns me.
I cant say from experience with vista or 7 or 8, but personally i would always prefer xp on a system such as this. the OS footprint will definatly be smaller, and the drivers for old/legacy devices are more abundant. unless there is some feature that a newer OS can provide that xp cant, and you cant live without it, i would stick with xp
be careful what you strip out too. i can't remember off the top of my head, but there are some components that are definatly needed by RR and or plugins or their dependencies (i.e. .net3.5) that need to stay in the xp install, otherwise you will spend tons of time having to reinstall windows with those components.
on xp bootup times, you can try searching for "minilogon" i believe, its from the XPe system, it will cut a lot of time logging into windows. it would replace winlogon.exe. you can also set a custom shell to be riderunner.exe instead of explorer.exe, but that can also present some problems depending on what functions you need