I dont know about "tougher", but I do know its a different profile, as are most bullets made for the 357SIG in that weight class.
Most standard 124 grain 9mm bullets start the radius/ogive to soon to work properly in the 357SIG. The 147 grain 9mm bullets dont have that issue as they have a longer "tangent" due to the weight.
Contrary to what you often hear, 357SIG and +P+ 9mm are pretty close in power. I got that very response from the engineers at Speer when I emailed them with the question on the difference when I was hot on 357SIG. They basically said both were 40000psi loads (Speer apparently has a spec for +P+ 9mm), and bullets of the same weight would perform basically the same. Thats why I was asking plouffedaddy if he'd consider a comparison of the +P+ 9mm.