Having owned both previously (and neither now
), my take on it is that the anti-.357 Sig folks are rather overstating their case.
Extra recoil? Seemed about the same to me going from a .40 to a .357 Sig barrel in the Glock 35 I had. The Sig is louder and has more of a "blast," but not by much. Both calibers are loud and snappy, and either one will leave your ears ringing if fired indoors.
Observation: .40 S&W practice ammo is almost all mild 180gr subsonic loads, while .357 Sig practice ammo is loaded up to at or near the same ballistics as the premium self-defense loads. Full-power 165gr .40 S&W loads are no nicer to shoot than full power 125gr .357 Sig loads, at least for me.
More muzzle flash? No. Muzzle flash is dictated by the specific load, with premium self-defense loads in either caliber having flash suppressants to reduce muzzle flash. Out of similar barrel lengths with comparable quality ammo the brightness of the muzzle flash was not much either way.
Comparing the hottest stuff in each caliber that you can actually get off the shelf (all courtesy of Cor-Bon's published figures):
9x19 125gr +P: 1,250 ft/sec, 434 ft-lbs
.40 S&W 150gr: 1,200 ft/sec, 480 ft-lbs (+10%)
.45 ACP 200gr +P: 1,050 ft/sec, 490 ft-lbs (+13%)
.357Sig 125gr: 1,425 ft/sec, 546 ft-lbs (+26%)
For those more interested in ballistic gelatin tests, if you compare the .357 Sig to the 9x19 test results at Ammolab.com, you will see that total or near-total (<.40") failures to expand when shot through 4 layers of denim were far less common with .357 Sig than they were with 9x19 (8/49 in 9mm Section 1 vs. 2/36 for .357 Sig Section 1).
http://www.ammolab.com/357_sig_1.htm
http://www.ammolab.com/9mm_section1.htm
Citing the performance of gee-whiz LEO only +P+ stuff in 9x19 to prove parity with .357 Sig is a bit of a stretch, considering how hard it can be to buy the stuff at all.
That said, I personally prefer 9x19, .45 ACP and 10mm Auto to either .40 S&W or .357 Sig.