I am going to be moving to DL. As a long-time EWR and then IAH-based flyer, I didn't have much choice, but since I have retired and moved back to Canada, it isn't such a lock any more. DL gives me somewhat better connectivity out of YEG, and their partnership with WS really helps things.
I just looked at their status match page, and noticed something interesting: it used to be that UA Silver matched to DL Silver and UA Gold to DL Gold, but now UA Silver matches to DL Gold and UA Gold matches to DL Platinum. I think this is a recent (last couple of months) change, I am sure it is in response to UA qualification changes: DL is trying to incentivize as many people to switch as possible by being more generous with their match status.
Note that matching from AA to DL is the opposite: DL gives a lower status level to AA flyers who switch. (I.e., AA Gold gets DL Silver, etc.)
The major negative is that DL matches 1K down to Platinum.