I agree with the others. As much as i love my 6.4 Hemi, and it has a lot of power, I'd love to have the Cummins. However, I'm not a fan of all the emissions stuff on modern diesels, so if your after more simplicity, yeah, get the gasser.
You do already have a paid for Cummins though and that would be hard to give up. Even with an even trade. There's a reason the dealer will go along with that. As another said, they will make out very well on the deal with your truck and they know it.
Regardless of what anyone postd here, you need to do whats best for you and your situation. Good luck