Hey there Dilvish!
I'm an empiricist at heart. So let's begin by taking the Trek Mamba 29er and the Trek 8.5 Dual Sport for example...
Trek 8.5 Dual Sport
Cassette: Sram PG - 950
Cassette: Shimano HG20-9
Personally, when it comes to suspended forks, I say, if you're going to have one, have a good one. Dual Sports don't have quite the caliber of suspended forks that quality MTN bikes do. The gearing on Dual Sports is pretty close to what you have on 29er MTN bikes. Of course, the DS gets the slight nod, on having the higher ratio for speed. There's not much difference there. The difference enters the picture, when you begin to consider the tire width. However, with more width, come more mass. Since inertia is mass dependent, a wider tired MTB is going to be slower on the take off, than a not so wide tired Dual Sport. OTOH, once that mountain bike gets going on rough terrain, it will be easier to control it, due to the wider tires and the quality suspended fork, working in unison, together.
I say, don't believe me, until you test ride them both. Besides, you've just got to try a 29er sometime during this lifetime anyways. Try the Marin Muirwoods 29er first! Of course, you already know that REI has an awesome 100% satisfaction guarantee. Apparently, if you either keep your receipt, or become a member and things go south with your bike, say...six years from now, you'll be entitled to either an even exchange, or a full refund.
Immediately after you've test ridden the Muirwoods 29er, try the Trek Mamba, so that you'll be certain about the 29er experience. Then take written notes about the experience while still fresh in your mind. Finally, try the Trek 8.5 Dual Sport. Make certain that you ride them all on either loose gravel or rough terrain. That's just for the experiment!
During the riding experiment, make certain that you also ride over terrain that will be comparable to that of your own, or at least terrain that will approximate it. Return home. Sit down with a nice fresh pot of tea and reminisce while you savour the flavor. Assess your experience and determine the bicycle of your choice.
I have no personal stake in which type of bicycle that you purchase. I only care that you get any new bike you want and have fun with it!
Good Luck, my friend!
If it feels good on the Muirwoods, the Mamba might feel just a tad too good!
* The Muirwoods does not have a suspended fork and you really don't need