From http://www.gunnuts.net/2012/06/13/38-special-p-vs-357-magnum/
Comparing loads side by side shows that .357 Magnum still has tremendous advantage in muzzle velocity over a .38 Special +P if loaded by the same factory. For example, the Hornady Critical Defense .38 Special +P is pushing a 110 grain bullet at a stated velocity of 1090 FPS, which would produce 290 foot pounds of energy. ... the Hornady .357 Magnum Critical Defense load is pushing a heavier bullet at 125 grains considerably faster – 1500 FPS according to Hornady. That gives us energy of 624 ft-lbs, which is…more.
Also, the .357 is a physically longer cartridge. You can load a .357 revolver with .38 cartridges (or .38+P), but not the other way around. (But you probably already knew that.)