You imply by using 2 x 500 milers that you're flying a segment of 1,000 miles or less. In this case, flying in E+ on UA is a good substitute for an F upgrade. The only real benefit you'll get in F on a 1,000 mile segment is free beverages and a bigger seat (personally, I don't see the in flight food - if any - as a benefit though others will disagree). E+ cancels out most of the benefit of extra legroom in F.

My personal rule of thumb is to save the 500 milers for flights of 3 hours or more and utilize E+ for the rest.