Playing the numbers game, and using bullet weights that are close, the 357SIG comes out on top....
All loads fro Double Tap's site (
http://www.doubletapammo.com/php/catalog/index.php?cPath=21)....
9mm +P 124 grain Gold Dots
Ballistics : 124gr @ 1310fps / 473ft. lbs. from a G17.
.40 S&W 135 grain Nolser JHP
Ballistics : 135gr. @ 1375fps / 567 ft/lbs- Glock 23 (4.0"bbl)
357SIG 125 grain Gold Dots
Ballistics : 1450fps / 584 ft. lbs. 4"bbl
Most of the 9mm loaded at the +P+ level and 357SIG, using bullets of equal weights, will give you similar velocities. Dont know why Double tap doesnt offer +P+ though, maybe its to close for comfort in their pressure ranges.
I use both their 357SIG 125 grain loads and their .40S&W 165 grain loads. They are a little hotter than most of the other factory stuff, but nothing noticeably drastic when you shoot them compared to Federals or Speer, etc. The 125 grain 357SIG Double Taps do bring you into the 125 grain 357MAG range.