Hmm. There’s not a huge difference in effective size in those two unis. I have a 26er with a 2.6" Gazz Jr tire, and a KH29er with an ExiWolf tire (2.2"). The 29er is clearly faster, but is currently limited by tire availability (esp. fat tires; there’s not much out there with big volume). The advantage of the 26 is that there are lots of fat tires (in the 3" range) you can use. Unless your used tires are pretty fat, I probably wouldn’t use them on a muni.
Depending on your size, the terrain you’ll be on, and how aggressive you plan to ride, each definitely has its advantage. If you’re not a heavyweight, your trails aren’t severely rocky or drop-strewn, and you plan on mostly straightforward riding, the 29er would be the way to go. It’s a nice uni, and the single-wall rim is the only real downfall, imho. But if your terrain is rough, you plan to be harder on the equipment, and speed isn’t your main objective, the 26er is a good uni. Personally, with either uni, I’d spend a little more and swap the cranks for KH (something like $40 more at time of purchase, iirc), as the Nimbus cranks are at the moment an unknown quantity.