I think the EVEN MORE SPACE on B6 will give you more room than a Delta Comfort+ seat. If your only concern is the seat, I would go with B6.

Personally, I try to avoid B6 because I find them to be less reliable AND if there is a MAJOR issue (weather or otherwise) you are completely screwed due to their non hub & spoke model. With DL, in most cases (if you know how to work the angles) you can usually find a way home. Granted - this is not a usual situation, but one I have gotten stuck with before and was pleased I was on a DL ticket