If you're buying new, the RAM is class of the field, GM is playing catch up. No doubt GM will introduce a much upgraded 2015 model HD -- there have been plenty of spy shots circulating of them being tested. GM's current interiors are badly outdated, and their powertrain falls short in the TQ department (at least on paper) when compared to the other two. The Duramax still is a good performer though, even when the numbers on paper are the lowest in the field. The Allison is proven and works great. If you like GMs, I'd wait until they update them.
Onto the mpg question and buying used (2010 - 2012), every test I've read has all of the big 3 relatively close mpg wise, but the Ram/Cummins (up to the 12 model) is almost always lowest. It is also outperformed in most performance based tests when compared to the Duramax and Ford diesels. That is why they really upgraded it in the 13 models (up to 385hp/850tq from 350hp/800tq). To put themselves further ahead, in 2014 they are adding air suspension and a new 6.4L Hemi gas engine among other improvements.
The 2013 RAM/Cummins also went to using DEF (as the Ford and GM have since 2011) which, although it adds complexity, does improve the mpg overall.
In the end, I'd suggest buying the one you like after driving them both. It's your money.