I think 15 wins for the rest of the season for the Astros is reasonable. And considering they're so bad, few of these wins will be blowouts, which means close games, which means save opps.
I think I'd go with Lopez because he is the closer whereas Affeldt shares the duties, and Affeldt has given up 3 runs in his past 6 appearances.
Or maybe I'm just a Lopez owner in denial
Either way, you're looking at a very close number of opportunities/saves. Toss a coin.