The Cummins is the most reliable of the big diesels, certainly in the long run, and I don't think it's all that close when you factor in the potential injection pump issues on the Dmax and Pstroke. Plus just the simplicity of the powertrain.
The Ram may have some issues here and there I guess, although if you look at the latest JD Power VDS the Ram slots in right between the GM clones and Ford in terms of dependability. I've had mine since last spring with no issues at all, although I know that's a small sample size.