If it's just a matter of picking one, then Invite them both over, then see how/how often they interact with your sim. The one that takes the most initiative wins.
If you really want a contest, then perhaps ice skating would work. For one, sims with low skill fall a lot, and that's just if they both continue skating long enough (you can time it through number of rotations or count the number of falls during a song). If one of them autonomously decides to stop skating, that in and of itself can act as an indication of forfeiture.
