Since you already ride a 29er, get the 26" for off-road. You will appreciate the little bit of extra speed.
Way back in my early days of unicycling (almost 5 years ago) my third unicycle was a 26" for off-road and it was more than I could handle at the time so I bought a 24" which worked for me. As time went by I bought a 29er and got good with it for on-road riding. Then I found I could handle the 26" very well for off-road. I sold my 24" and have been riding the 26" since. I have very long (175mm) cranks on my 26" but have been thinking of trying some 150’s.
I would get a 24. I don’t know of any cheap, strong 26" MUnis, but the 24" DX works for a good price and I think more 24" MUnis are available on UDC for close to the price of a DX, with ISIS cranks etc. A 24" is plenty fast on a trail and is still somewhat light (except for the DX hub).