I just had an ISIS wheel built for my 36-er and have KH 125/150’s on there. I am finding that freemounting is quite hard with the 125’s; I live on a hill and I can’t freemount uphill.
It’s going to take a little getting used to but I do like the shorter cranks (I had 150’s on my other wheel). MUCH less knee movement and I have the ability to “keep up” with my cranks when going downhill rather than slowing my 150’s down. It’s amazing how much 1" will do with the torque and control involved.
I don’t have a 29er but based on my experiences with 36ers and my 27" (32x630) I think that 114s would be a great size for road riding and easy trails.
As far as I know Qu-Ax makes the cheapest 114mm ISIS cranks. I have a pair that I have not used yet but I have been happy with their 125mm and 145mm. cranks