I have ridden both 29 and 26 recently. My first go on a 29 was a fully rigid single speed and it was brilliant! so fast up hill, rolled over all the roots and rocks I thought might send me over the bars no problem and generally coped with everything I thought I needed suspension up front for. I was impressed. It was bloody harsh though.
Then I got a go on a Yelli this weekend and went for some fast times on various bits of trail I know well (Strava content beware!) and I went faster everywhere but in all honestly I was trying harder and it was a perfect weekend for it! It went over rocks and roots nicer (Not necessarily faster) and cornered well with predictable slides where I thought maybe a 26 would have washed out. This was first thing in the morning and was fun but not wow this is the future fun..
That evening i went and did the same trails on my 26 (C456) and was near as damn it the same time down everything, I can’t say I noticed much more harshness in ride and got to the bottom just as out of breath as the 29.
So in summary. I’ll most likely get a 29 single speed rigid for most of the riding round the surrey hills and hold on to the C456 for everything else.
Not tried a 650b so can’t comment.