Join Date: Jun 2010
Location: Cleveland-ish, OH
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I'm far from an expert on this, but I think you'd be much better served by purchasing a bike that's right for the sort of riding you plan to do 90% of the time and then either rent or borrow a bike for that other 10%. That said, maybe there's a singletrack trail you didn't know about not too far away. I was surprised when I found the one near me, and I hear there are a couple more hidden nearby.
Travelling with a heavy bike from Illinois to Colorado won't be cheap and easy, so that's something to consider too. Also, note that the Mamba has 100mm of travel on the front fork and the Utopia only has 63mm - that's a big difference if you're planning to jump off things onto other things.
And since they both have lockouts, you can pretend you have a rigid fork for the normal rides, which would be nice. I'd at least consider a second set of tires, even if you don't want a second bike. IMO, gravel trails do fine w/ smooth tires, maybe w/ a little grip at the edges, but you want knobby for snow and mud and dirt.