A LOT of factory .357 Magnum loads are watered down, as people don't like firing the hot stuff out of K-Frames that get rattled out of time and airweight J-Frames that split the web of your hand.
It's gotten to the point where a lot of people consider a 125 grain bullet at 1450 feet per second to be a super "hot load" for .357 Magnum. Now, I haven't shot a lot of .357, but if you look at reloading manuals and see what .357 can really do, you'll find that 125 grains at 1450 really isn't all that hot. (If you think it is, trying doing that same velocity with a 210 grain bullet through a .41 Magnum. Now that's entertainment!)
Of course, I've heard that these .357 loads generate excessive amounts of noise and flash. Perhaps they feel hotter than they are?
In any case, .357 Sig is marketed as duplicating the ballistics of these mid-range 125 grain .357 Loads, which it does well enough, from what I've read. I've heard that .357 Sig guns have shorter service lives than comparable .40 models, but it's second or third hand information.
In terms of raw power, though (pushing the heaviest bullets to the highest velocities), .357 Magnum spanks .357 Sig quite handily.
However, 158 Grains at 1400 feet per second might be too much recoil for you, or another shooter. There is a point of diminishing returns, where recoil becomes so great as to actually hamper shootability. For me, .41 Magnum and warm .45 Colt are perfectly comfortable, as is .45ACP +P and Super. For another, 125 grains at 1400 feet per second might be all the oomph they're comfortable with.
I've heard that the 125 grain .357 loads sometimes fragment and don't penetrate deeply enough, and the same consideration thusly applies to .357 Sig. However, given proper bullet construction (or, as a last resort, use of full metal jacket ammunition) this wont' be a problem.