Just based on the differences between 26" and 29" wheels, it would be of benefit to consider the style of riding you prefer. I'm not personally familiar with Diamondbacks (at least not the bikes...I know more about the snakes
); but,
in general, a 26er will handle more nimbly and be easier to "jump" and throw around on the trail, sometimes requiring a more technical approach, while a 29er will keep its speed and momentum better, and have an easier time rolling over obstacles in a way that tends to inspire confidence. 26" vs. 29" can, to a point, be boiled down to "twitchy" handling with a more technical ride on the 26er vs. greater momentum and ease of clearing obstacles at the expense of some of the "playfulness" in the handling on the 29er.
If ether wheel size would suit you, then take a closer look at specs and componentry, but that wheel diameter will most likely be the biggest differentiating factor between these two.