The geometry isn’t wildly different- 67 headangle as opposed to 66.5, although the Enduro has a 20mm longer wheelbase and a 15mm taller BB. Chainstays are 1mm shorter on the Enduro. TTs are the same.
When you compare the geo on a 29er 2015 Enduro v Stumpy Evo, it’s a slightly different story. Wheelbase is the same for both, chainstays are 25 mm shorter on the Enduro, TT is 5 mm shorter on the Enduro, BB effectively 10 mm higher on Enduro (corrected for sag), head angle 0.5 deg slacker on Enduro.
Main reason I went for the Enduro 29 over the Stumpy 29 in the end was because of the shorter chainstays (they are very short for a 29er). Reading between the lines it seems Spesh haven’t got round to shortening the Stumpy chainstays yet, although they may have just decided longer was better for climbing.
It’s all a compromise of course and while the Stumpy is almost certainly a better all-rounder for most UK trails, I thought I’d enjoy the Enduro more. I usually ride alone or with my wife who is a bit slower, so I don’t mind a bit of extra weight to lug around – not that it’s particularly heavy for a a trail bike (29 lb I believe for a large in carbon, but haven’t weighed it yet)
First impressions are great. Climbs fast as far as I’m concerned (29″ wheels?) and obviously more than capable on the downs. Climbing switch on the Cane Creek seems to work really well – rear seems to sit higher in travel and no pedal bob, but still feels reasonably compliant over bumps even when forgetting to switch it off for the descent!
OP – you should really try a 29er Enduro before making a final decision on the 650b. You might be pleasantly surprised how good an all rounder the 29 version is. For pure techy DH the 650b may have a slight edge, but for everything else I suspect the 29er is better.