The most plausible conclusion to me is a draw or the heroes distracted elsewhere...how can you have a "realistic" conclusion to a match between two beings that are more impossibly fast than the confines of a planet like Earth can even accomodate?
I would say that yes, you can have a realistic conclusion.
Realistic means something quite different in science fiction than it does in other types of stories. It means creating rules and being consistent with them.
In the Star Trek television show, the Transporter was an obvious piece of fantasy, working by really, really mushy physics. But we were made to believe in its existence because they created rules as to how the transporter worked that made it feel plausible. The transporter can't beam through forceshields, the transporter requires the ship to be at full power, and so on. And, if the characters are faced with the likely possibility the transporters may not work, they still have shuttlecraft - another nod to how well the writers of the show thought this through, since it would be illogical (no pun intended) that the transporter, just like any other piece of machinery, would always work, and so gutsy folk like our space explorers would anticipate this.
Superspeed is an obvious departure from reality, as are superpowers. But they are made to feel real because rules have been created as to how these powers work, and the details are defined as to what these characters can do. And with these rules in hand, we can make a pretty shrewd guess as to the outcome of something like a Superman/Flash race.
As in most things, for example, a team that wins a match or a seven game championship series, etc., I give credit to the winner without assuming he/she or them are "better, faster, stronger, more skilled" except under those circumstances...
You're very correct and right. Heart, guts, will, and just plain luck are all factors that give victory in a sports competition to one side or the other, and may be more important than just plain statistics and batting averages. But Superman and Flash are both great, heroic superheroes; they've
both got heart, guts and will. All things are equal here, so one ought to look to external factors.
Also, a draw, as I have said before, is unsatisfying because there is no closure and doesn't resolve anything. Not that resolving anything should be the main priority of a Superman/Flash race; telling a good story ought to be the main priority. But we can have our cake and eat it too: it can BOTH have a good Flash/Superman race, and tell an entertaining story as well.