Kinda a hard question and I dunno how you would test it. The Vista bootloader doesn't work with the XP one and vice versa (i think). I'ld *guess* the Vista *bootloader only* is faster. The Windows xp bootloader opens autoexec.bat which is now redundant and isn't included in Windows Vista (and later) and being an older bootloader isn't as optimised as the Windows Vista one.
This is only my opinion... hopefully someone who knows more than I do will share their 2 cents (or 2p if your from the UK like me ).
... thats about the speed though. I don't know much else about it. I read some stuff on the MS blog site when they were making Vista that they were optimising code so it works much faster depending on the Hard Drive. They said it can load more MB in less time.. doesn't mean that would make Vista boot faster than Xp though (though it does on some computers) as Vista is a much more comlex and larger OS than xp.
... hmm I'm remembering more now... the vista 64bit boot loader is more secure than 32-bit as it loads system files from random memory locations rather in a sequenced event making it harder for a boot virus to be effective.
Also, pressing F10 at boot (like F8) brings up a memory test screen to test RAM to help booting up on vista and the vista boot loader is easily fixed (most of the time) using the Vista startup repair tool... this just wasn't around for Xp.
Thats pretty much most of what I can remember at the moment. I'ld write more but I think I've writen enough.
oh and they mentioned something about a balance between loading compressed files vs uncompressed files depending on the CPU power... a slow CPU would take less time booting larger and UNCOMPRESSED files where a fast CPU would take less time booting smaller and so COMPRESSED files... that kinda stuff was put in use for the Vista bootloader.