If the history of airlines that have gone into bankruptcy proceedings in the past is any indication, Delta miles will keep their value, so I wouldn't worry on that score.

I think airline miles are more valuable than PC points. If you redeem PC points for a reward night you get a bit under 1¢ per point real value: for example, you wouldn't have to spend $250 for a room at any Crowne Plaza I've ever seen, though there may be exceptions if you pay rack rate in a major city. Ditto for their other hotel chains. If you redeem for an "any hotel, anywhere" prepaid certificate it's worse: from 0.3 to 0.4 ¢/point depending on the certificate value. If you want an airline award, it takes 4 PC points to get an airline mile. Looking at how many airline miles an F award to Europe requires, and how many $$ or PC points it would take, it's a no-brainer.

The only question is: will you spend enough on the card so that, along with DL miles from other sources, you'll have enough for that ticket? If not, PC gives you useful awards at lower totals. Stick to hotels in the Reward Nights program, and choose carefully for best value.