I guess I don't really understand your question. An open seat on the flight is an open seat. On DAL your chances of either seat are the same... it's not like some airlines where non-revs can't get First of have to pay a fee.

From what I've seen in my 8.5 years of Delta non-revving (5 as a commuter) is that there are usually ample Medallions standing by for an upgrade to fill First. This opens up coach seats provided you're high enough on the standby list to get one of them.