Xtracycles are pretty nice, but Yuba sells just frames. The Yuba frame + all the parts off your current bike (assuming it's not a '29er') would be cheaper than getting another bike to attach it to. You definitely want 26" wheels, they are massively stronger than 700c, especially when paired with something like 2.35 Big Apples/Fat Franks. Well, I should say, it's not that 700c are that much weaker than 26" wheels, it's more than it's pretty much impossible to find 700c wheels with more than 36 spokes, while finding 26" ones with 48 spokes isn't all that hard to do, and a 48 spoke rear will definitely also help stiffen things up when loaded.

I'm still saving for a Yuba myself, but my current ride is an older Cannondale 3.0 frame MTB from 1992. It's got huge diameter aluminium tubes and when I load up the rear rack I can definitely feel it flex, and it's without a doubt a much stiffer frame than either of the two bikes you mentioned. I don't think you'll get any appreciable difference by swapping the base bike. Either save up and get a pre-kitted Yuba, or swap all of your current components onto a Yuba frame for a good bit less money.