I had the same option back in the day. Wife and I were heading back to the midwest after military time in Utah and United was the choice because of Chicago (before NW merger). I have a lot of friends at Delta, and as pilots, we all tend to gripe a lot about our different airline related issues, but you truly cannot go wrong at either airline right now.

A wise Captain once told me, "You will never know how your airline career will go until you shut the throttles off for the last time."