First you must set your priorities. Do you want a good carry weapon, to defend yourself, or do you want a good competition weapon, to score high?
For high scoring in competition in IDPA, you appear to be talking about non-CDP weapons, which means you would likely be looking for the lightest recoiling weapon you can find that meets the power floor, fits the other weight and size requirements, fits at least 10 rounds in the magazine (more is irrelevant in IDPA), and so on. The 9mm would be a much better choice.
For carry to defend yourself, you have many more variables to consider, including momentum, kinetic energy, ballistics, your own comfort with recoil, "stopping power", number of rounds carried, and so on.
Personally, the only reason I see to carry a 9mm is because you're buying a Rohrbaugh due to the size. Anything Kahr size or larger, and I want at least a .40/.45/.357 Mag. .357 Sig might be OK as well, though I dislike dealing with bottleneck cases from a reloading perspective.
For both you have the usual cost of ammo, practice, and fit in your hand considerations.