This may be heresy for a mileage board, but I think you're putting the cart before the horse. The first question is where you fly, who serves those destinations best from where you are, and is their FF program good enough to be acceptable. Only if the airlines are close, or if one of their programs really sucks, do program issues come into it.

Another question is the kinds of tickets you usually buy. On Delta, you can't upgrade most discount economy fares. United has no such restriction. If this matters to you, points to UA, especially since you'll get increased legroom on any fare once you're elite. If you usually pay full Y or better, points to DL: less competition for the front cabin, so their tight seating in back won't matter to you.

If there was one right answer for everyone, everyone would be on the same airline - and then it wouldn't need a frequent flyer program at all!