OT: which derailleur do i need?

[vqdriver]

on my mountain bike, i'm switching to shimano trigger shifting from sram.
if i run the XT rapidfire shifters, which shadow rear der do i need. planning on xt for both.

i didn't know there were so many variations until i started shopping. depending on which site i look at, there's hi normal, low normal, sgs, sg.... wth?