Given DL's seat capacity caps, I'd opt for the E175 if you can. While DL is no longer blocking all middle seats or blocking all one seat in all 2-pair seats, DL is still blocking B-seats on the A-B side of the CRJ fleet and the E-175 fleet, which alone would be enough of a reason to go with DL. Better social distancing practices on DL than UA right now, at least if you can book and select an "A" seat on the E-175.