2013 Trek 520, 2014 Trek Stache 8, 1985 Nishiki Team Issue, 2014 Dahon Vigor P9, State Vice 2.0
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.
2007 Kona King Kikapu - 2012 Raleigh Misceo Trail - 1993 Diamond Back Sorrento - 1989 Nishiki Blazer
If you have to decide over the two, I'd go with the Overdrive. The two are basically the same, I'd give the Overdrive the advantage of having 29" wheels. You may be able to get a little more if you sell it later on. I also second the craigslist idea. I used the site "searchtempest.com" and looked for bikes in a 200 miles radius. My city didn't have the greatest selection, but surrounding ones did. I ended up driving 100 miles and picked up a great deal...just a thought.
I'm really considering the 29, hard. The only problem is I've never ridden a 29, and I don't know how different it would be. I like the easy of pedaling a 26. I'm not sure how different it would be on a 29. Are the 29's more sluggish when accelerating?
I would recommend the Craigslist route. Just keep watching...you'll get a far better bike for the money that may only be a few years old. Those new diamondbacks just aren't worth it in my opinion. They don't look like much more than your walmart special. They will probably function just fine but that bike compared to a used mid range bike will be worlds different.