I think try them both, they’re surprisingly different- for me the 29er’s better but that’s going to be a personal call.
The 29was a 9-spec not an 8 so a fair bit nicer. The test loop was glentress falla brae so nothing very techy or tight, but a reasonable testtrack still, some rocks and jumps and wiggles and drops and roots and that. I’m going to try and nip up to nevis range next week and see if I can try them on the world cup, better test 😉 The other comparos were a really nice Five 29er and a Whyte T129 so all good bikes, well specced, stiff competition. And my own Hemlock which I love to bits.
IMO 29er was better at absolutely everything except maybe jumping- my jumping is just “get to the other side”, nothing clever. Can’t fairly judge, not my thing.
The 650b just felt a bit imbalanced and, not unstable exactly but uncomposed and nothing like as confidence inspiring, brasher over rough stuff and generally harder to manage. The same thing that makes it fun to pop and jump and manual I suppose.
The 29er, OK less fun, less playful but more grip, more stable, not noticably bargey or cumbersome or anything- it didn’t feel like a massive bike or like the old idea of a 29er (neither does the Five btw), I could put it exactly where I wanted to go with no real effort. Massively confidence inspiring and planted. Matched my Hemlock to the second on the main descent bit, despite being a first run vs 1000000th run 😉
Nothing wrong with the 650b I don’t think, good bike. Just not superb, 29er imo is better than the 650b and most other things.