I’d rather buy a Giant, but if 24hr racing I’d go for the 29er.
FWIW I’ve always had SRAM shifting on all my mtb’s (mostly X0 and X9) and I’ve never had any issues with it (other than the old X0 carbon cage rear mechs that had a habit of exploding.) Though chainsets have always been XT or XTR. The Giant comes with Avid’s which is enough to put a lot of people off, though easy enough to change.
Edit… I’ve used both SRAM and Shimano (comparing X9 and XT) and the feel is different. If you go to SRAM from Shimano then it’ll probably seem heavier and clunkier and maybe not as smooth as XT. I like it though as it’s a very definite shift. They both work well IMO but the feel is definitely different.