If most of your miles come from Marriott points transfers then you're probably not flying much, maybe enough to reach Silver if any status at all. So the status upgrades are probably not a benefit that would be meaningful. If most of the travel is booked as award travel then you're not really going to be a hub captive and can fly any airline that services your airport.
With that in mind, the question really is "how much time do you want to spend looking for your award travel and how much help do you expect?" Redemption rates for US on US metal are either really cheap or really obscene, and never in between; CO's seem to be a little more sane but still not precisely "cheap." The advantage to flying on the same carrier that issued the miles is that it is easy to book online. The Continental site itself is not too user-friendly for more advanced routing though. Continental will show you some LIMITED selections on partner airlines on their website. Redemption on partner airlines is higher on CO than on US, which makes US more attractive, however US will not show you any partner availability on its website, leaving you to do the work. If you know how to do it and where to look, you can find some stellar partner awards for US mileage at pretty attractive rates. But it does take some homework and it can be frustrating at times.
If you plan to redeem the awards with the airline that issued the miles, then CO is probably the way to go. Their first class to Hawaii is a lot nicer than US' and they have more choices when going to Europe. If you don't mind the homework, and don't care which carrier you fly, then US is the way to go, as their Star Alliance awards are really the best part of the DM program.