I think it will depend on what hardware your running on. 98 will do much better on crap hardware and XP won't be able to touch it, but I think XP will be a bit faster then 98 if you give it enough memory and processing power.
Why is boot time even an issue though? Why not just use hibernate which will probably be twice as fast as your boot time.
I did my first carputer with 98se and second with XP and find XP much better if for no other reason then its easier to maintain and you don't get bluescreens.