I’m currently riding a Trek Rumblefish 29er with 120mm at each end. In comparison, the last full suss 26er I had was a Remedy with 160mm at each end and Fox 36s. The similarity between the pair is uncanny, apart from the fact that the 29er is better! Yes, it comes in at about 31lbs but it’s not built to be an XC racer (nor am I, for that matter).
I can ride stuff more aggressively than I’ve ever been able to on all my 26er trail bikes, the big wheels just eat everything up in their path. The climbing and cornering grip is also phenomenal.
If money were no object, I’d compliment it with a 29er 100mm hardtail for racing, but unfortunately that isn’t the case!
In short, a120mm FS 29er is a LOT of bike. If you want something for purely XC, maybe try a 100mm set up first?