Except that DL isn't counting spending on partners either, unless you buy the ticket as a DL codeshare. As for your original question, I like DL better than UA out of BOS due in no small part to the fact that domestically, I don't have to connect through CLE, EWR, or IAD...CLE is an RJ farm and the other two are disasters (though IAD is getting better.) I also find DL's hard product to be very good with Wi-Fi on all planes and TVs at every seat on many. DTW is a great place to connect and I personally have no problem with ATL (though I know others do.) I do avoid JFK if I can.

One major downside of DL vs. UA internationally is if you buy coach fares and try to upgrade. It's much harder to do that on DL. And of course it's a heck of a lot easier to redeem UA miles.