I think skypesos are worth less than UA miles -- they are more difficult to use, go to fewer places because DL has fewer good int'l award partners, and DL plays rather irritating games with award availability. But the southern charm and service of DL (or at least PMDL) is better than UA's hostility, and the upgrades I have found to be totally worth it.

That said, I moved recently from NYC to BOS. Whereas DL was better than UA and CO at NYC (I prefer JFK to EWR) it's a closer call at BOS, and I am considering a status match to UA/CO.