I prefer 124 for the reason 9mmepiphany stated. 115gr is on the light edge of 9mm bullets, 147gr is on the heavy edge, 124 is right in the middle and is what most 9mm pistols are built around. I have lots of 9mm ammo of various loads, but I would want to carry the 124gr(+P JHP) or the 147gr(+P FMJ-FP), depending on environment/situation.
I agree with your choice to go with 124gr, however if you have both 115gr and 124gr - and it's all quality ammo - I don't see any problem with carrying either one, particularly since you're familiar with your pistol's handling with the 115gr.
It'll all probably "do the job" well enough unless you want to carry in the woods.