If you had a rigid 29er or a 26 with a 100mm RS SID, overall weight the same and all the rest of the spec identical.
Lets say the race is a Gorrick or Southern XC series.

So what would you choose and why?

I’d spend a few hundered quid at Xc racers shop and take the 26 w/100mm sid. But you should take the rigid bike, as they way I’ll be in front of you. 😉

Unless you’re actually going to be troubling the top few places, just take the one you enjoy riding most.

The rigid 29er. Then I’d have an excuse…

I’d take the rigid 29er… but that’s what I ride all the time anyway 😀

And throw the gears away, then you’ll shave a bit more weight.

Irrelevant it’s the engine that counts 😉

I’d take both, recce the course, and ride the one I think I’d be quickest on. Probably base it on how gnarr the course is and how much harder riding a rigid fork would be.