If you’re talking just decent MTB (No £99 Specials) then its pretty much Shimano in the low end, and SRAM at the top. Of course it helps that neither SRAM does that many very low end forks (at least from a cost perspective) and that Shimano doesn’t do much decent stuff other than Drive and brakes at the high end.
If you look a the value of the components on a bike today, at least something high end MTB, then SRAM can often account for 60%+ of the component value of a bike, usually some combination of:
Forks
Reverb
Shock
Drive
brakes
Wheels
Whereas shimano will be limited to around 10-15% of the value in the drive and brakes only. Add to that the fact that SRAM is definitely increasing in both popularity and OEM coverage, while Shimano is at best static, in MTB world SRAM is most definitely the bigger player, by some way.