Here's how the components break down, roughly, comparing Shimano road to Shimano Mountain to SRAM mountain:

Dura Ace - XTR - X.0
Ultegra - XT - X.9
105 - LX - X.7
Tiagra - Deore - X.5

Though I imagine you won't be looking at anything below LX/ X.7. Different people have different prfeerences, of course, and different groups have been updated mor eor less recently, but those are some rough equivalencies.