I've hardly ever flown either, so I hope you get better-informed responses than what I can offer. But the FlyerTalk wisdom seems to be that Delta is generally a better airline. Part of the answer to your question might involve where you want to fly with the miles you accumulate and whether those destinations are better served by either airline (and their partner airlines).

Just in case you don't get many responses here, you could either simply add a post to this thread asking the Moderator to switch the thread to the Delta forum. Or if you go a few days without good answers, you could re-post on the Delta forum yourself. Posting the same query on two different forums is an FT no-no, but I would think and hope that it would be understandable if your first post doesn't yield good info and your other post might.