The OP said it - it's a no brainer.
If only one calls you the choice is pretty easy.
What if UA hires him and a month later he has a DL class? Or two months later? Six months? A year? At some point jumping to DL makes sense and the longer the delay waiting for DL's class the tougher the decision gets to make the jump.
There are people in this situation - already hired by their #2 choice and have job offers from #1. Every airline has hired guys from, or lost guys to, other airlines. And some have probably made the decision to stay with #2 because of the lag to getting on #1's seniority list.