Since you can get a 9mm in a smaller platform than any 9X18 I know of, why would you eliminate the 9mm?
Some people find guns more shootable when there is an optimal fit between the size of the gun and the power of the cartridge.
Not me, though. I'm looking for a gunsmith to design my dream CCW piece: The .50 BMG Ultra-Micro Carry. If it's all about getting the most powerful cartridge crammed into the smallest possible gun, I'm not messing around.
As for the original question, I don't think it makes any practical difference. Find the gun you like better, then look on the side to see what it's chambered in.