By all means, make sure the gun you choose is one that fits her hand and for which she has sufficient hand strength. Also, if she's willing to spend the time to master the auto's mechanics, that's fine; if she's not, there's your answer. In short, I agree with everyone above: take her to the store, have her handle the guns extensively, and see what that shows you.
My wife is very handy with tools and unafraid to shoot, but a K-frame revolver is almost too large for her to pull the trigger. A J-frame S&W or D-frame Colt, however, is fine. Rugers bring a different frame size & often a tougher trigger pull--my daughter couldn't pull the DA trigger on a new SP-101. . .until I replaced the mainspring with a Wolff spring ($10, 10 minutes). My Beretta 92F was too large for both wife & daughter to handle, and I saw many females in the Army for whom it was a difficult fit, too. Also, my wife & daughter can barely work the action on the 92F, and it is far tougher to handle the auto than the revolver.
As they say, your mileage may vary. I've simply learned--the HARD way--that my theoretical analysis tends to overlook something unexpectedly critical, and I will never again invest in a gun for them without having them check it out.
As far as effectiveness goes, .38SPL and 9mm are effectively equal.