Most of the units guys have installed are the D4. I have the D4 and find it to be sufficient for my needs. I live in Idaho and at times it can get fairly cold. I have had no problems with mine. I set the timer to let it run 1 - 1. 5 hours before leaving and the engine and cab are always warm.
There was a guy from Canada posting last winter that he had the D5 installed and it sounded like it takes less time to hear up, but I have no personal experience with it. I believe it is quite a bit more money, and the kit Espar markets for the Ram is the D4. The kit is very well thought out, and complete. Istallation takes the better part of the day, but not too difficult for someone with a good selection of tools and some mechanical know how.