While I have nothing to base this on other than my mechanics recommendation, I use startron. I'm sure that they both have similar properties in keeping the fuel "fresh". However, since the E10 gas is around, I have greater comfort in how Startron claims to protect against water separation. Stabil claims to just be a fuel stabilizer.
Of course, Stabil now has a marine formula, which may make the decision more difficult. But for me, I'm sticking with Startron until I'm told something different.
David