The difference, ballistically, is a wash, if you're not too swayed by 'energy' calculations (there's a good case to be made that plain old momentum is more important in SD ammo). From a 4" barrelled .38, you could push a 125 gr. JHP +p at about 1000 fps; a 9mm will give you a couple hundred extra fps from a 124 gr. bullet, but at HD ranges, it's not gonna matter. Or, if you go heavier, as I prefer, you can get 890 fps from a .38 LSWCHP +p at 158 gr., or 980 from a 147 gr. JHP in 9mm. As I say, a wash.
Of course, if you're limited to less than a high-quality semi-auto, the choice goes to the revolver on reliability grounds. But if you can get into a Sig, HK, Glock, or something else that reputation and experience show you is utterly reliable, then it's just a question of your own shooting preference.
Unless you're married or otherwise cohabiting. I shoot a Glock much more regularly than my 686+, but the revolver is the one that stays handy in a Gunvault, 'cause my sweetheart's a little more comfortable with it, and I do fine with it, and 7 shots is enough to get the family rounded up and get the shotgun out of the big safe.
Sure, some freak-out circumstance could make you want an extra 4, but if you're losing sleep over that, you need another hobby
Good luck, and enjoy shooting the snot out of whatever you go with.
CG