I had a blast on a 29er, but the bike felt out of school on tech-knar at times. I would never consider crux moves such as skinnies to a wheelie drop etc. Mind you that's 0.5% of the time. I was too high up to feel confident on balance moves. I'm old-school and like that stuff. My build was light, and flexy in the front (fork and wheel), and had a steepish head angle so that all played a role. But I do remember doing some big stuff on my TBLTc. It was a confidence inspiring rig with any little bit of momentum. For slow, hard braking, tacking through a section, not so much.
The Prime intrigues me though, now that I know how good the KS link is (and DB air). You could always go industry 9 or something for wheels. Or get some LB carbon rims, and build them to hope hubs. Not a huge investment.
For me the deciding factor would be planned mileage. If you cover long distance on a day out, go 29. If you like riding the harriest stuff, go straight there, get your adrenalin fix, then head back to the truck, go 650b.
My perfect quiver of 3 would be my 26er DH bike, my 650b shore bike, and my Squamish 29er. But I've made a commitment to myself to stick with two bikes because they are enough work themselves.
The tricky thing with choosing the 29er will be sizing. For reference, my XL TBLTc fit like a glove, and on paper its a small bike relative so some XLs out there.
With my rune build I went 650b with an Xfusion Metric, running it in 180mm. Its been a blast to ride, and climbs great. XL frame with 35mm stem, borrowing from the 'forward geometry' concept. 35mm is twitchy in the slow, but really fun with momentum.