Honestly, I think the 124GDs should be just about right for both.As far as ballistics go.
I doubt you will find all that much difference within only 1 inch or so of barrel-length difference.Maybe 40 -60 fps with the same load in both? If even that much.
It's 9mm, and under 200 grains, so I wouldnt even worry about "energy" levels @ impact, a moot point, at best.
But, given the handicap, 124s should sort of "split the difference" for velocities-vs. 147s.
If you havent tested penetration of your rounds, I'd suggest at least trying to do so, 1st. -Simple and quick, right out in the back yard.- 6 x 1-gallon water jugs end-to-end. With something behind the 6th as a back-stop. Your desired pen. should be around 4-5 jugs,whether expanded, or not.
If you hit the backstop, then expect over-penetrations,of course.
And then there's the JHP via short-barrel thing... in 9mm and .40, JHP really dont often expand anywhere near as much as marketing dept.s and internet forum mythology would like anyone to believe. Even with "modern" "quality" HPs.
The proof is in the pudding, so test it to find out. If you do test, and you get the min. 4 jugs penned, without expansion, then just go ahead and save the $$ and get some good FMJ/TMJ (blazer brass works very well).
If you only get, say, 2-3 jugs penned, then forgot JHP, completely, and go with 147 FMJ/TMJ.