The two cartridges are basically the same, shooting the same size/weight bullets at the same velocities, so the only real difference will be in the pistols.
As far as recoil goes, using the same cartridge, a lighter pistol will kick more/harder than a heavier pistol. In addition, the frames of these two pistols are not the same size when it comes to grip circumference. The G37 has the same size frame as the 9mm/.40/357 Glock pistols; only the slide is heavier and slightly larger. Most folks feel that a pistol kicks less if they can really get their hand around it and control it. So...
The G21SF weighs slightly more (empty or loaded), and therefore should kick slightly less, based simply on the math.
The G37 is smaller and easier to grip, and probably sits just a hair lower in the user's hand, both of which will make it "feel" like it kicks less.
If it was me, I'd go with the G37. I kind of made this call a couple of years ago, when I rather unexpectedly found myself owning a G21SF. I used it for a year, but simply could not get over the larger grip size, which was different than all my other Glocks. So I sold it.
I never bought a G37 for two reasons: cost of the ammunition, and the general lack of availability of the ammo. When I could find it, .45 GAP ammo was 10%-20% higher priced than .45 ACP, and I've never really been a "gotta have a big bullet" kind of guy, so I just bought a .40 caliber Glock (first of several), and that was that. I now have only 9mm and .40 Glocks, and I don't feel restricted at all.
Hope this was helpful.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)