arent .357 sig's less powerful then .357 magnum desert eagle's though?
Well, there's a blending of ideas and specs ingrained in your question.
ATLDave pointed out earlier on:
The .357 Sig can almost duplicate ONE .357 Mag load - the 125 grain light for caliber JHP. It cannot do any of the other stuff a .357 mag can do.
So when you are talking power, you are talking about bullet weight and velocity. The 357 Sig cartridge was designed to mirror or at least get close enough to the 125 gr 357 magnum loading that was used by police for many years in revolvers, and put it into a gun that could hold more ammo in a smaller package. I'd say they succeeded, but the importance of that success is very debatable. So if you are looking
purely at that bullet weight, the 357 sig comes close to mimicking the that one bullet weight loading in 357 magnum. The magnum is what the Desert Eagle can be had in. The Coonan Arms 357 pistol also shoots 357 magnum, and not 357 sig. But I believe if you look at load data, the magnum 125 gr loads can still be pushed faster.
If you look at 140-200 grain ammo in the 357 magnum, it may have lower ft lbs of energy compared to 125 grain loads, but that's just because you aren't going to be able to push the heavier bullet fast enough to match that energy level. The length of the bullet limits the case capacity, and increases pressure. However, you are likely going to get better penetration with the heavier bullet even at a little lower speeds. So when you really break it down, yes the Desert Eagle and Coonan shooting 357 magnum ammo could be considered "more powerful" than anything chambered in 357 Sig..
But what does that really mean? Choosing a bullet weight at an appropriate velocity for the task is what matters, and "power" becomes a relatively unimportant idea, because it doesn't really tell you anything. Yes, you can blow a big hole in someone or something with a 357 sig, and that load will definitely stop a human based on the numbers 94095 lists in his post above. It's a good cartridge. But you will also get a lot of flash, and noise to do it, and shooting it indoors would be very very loud. Any gun would, but I've heard 357 Sig is quite noisy.
On the other side, a heavier bullet fired from 357 magnum will penetrate deeper in a heavier bodied animal, or may defeat barriers your attacker is hiding behind. So really, "power" doesn't tell you a lot about the cartridge or how a bullet will interact with a living creature. It tells you a general number which is a unit of work, that gives you some idea of what combination of bullet mass and velocity you are getting. But where does the extra power go if you shoot through your attacker? It goes into whatever is behind the attacker. So is the 357 magnum better than the 357 Sig? It depends on the application. Is it more "powerful" yes if loaded properly it can surpass the 357 Sig. Does that matter? Maybe depending on your needs.