They are different. The powerbelts are actually bore diameter where as the shockwaves are a sabot's so thy are actually a 45 cal diameter for a 50 cal gun . They both are good shooting rounds however i prefer the shockwaves because the powerbelts left a lot of copper fouling in my rifling. I did some research on this and it has to do with the way they electrostatically adhere the copper over the core.
The shockwave is a saboted bullet. It is a 45 cal bullet in a 50 cal sabot. The powerbelt is 50 cal bullet with plastic shot cup attached to the back end. They each have their pro's and con's. They will both kill a deer shot through the vitals out to 100 yards. As far as which one is better? It depends on which one your gun shoots better. Every gun is different you have to find the bullet that yours likes the best.
In my personal opinion there are much better options available than either of those two. I like both the Hornady SST's and XTP's. My muzzy shoots both very well and both bullets have great terminal performance. I have heard nothing but great things about the Barnes Expanders but have not yet tried them myself.
I am running 295gr Powerbelts in both my TC Scout, pushed by 100 grains of GOEX loose powder and my Wolf using two Triple 7 pellets. The Powerbelts group like a charm in both rifles. I like that there is minimal plastic associated with the Powerbelts, this allows me to get 3 or 4 shots fired before having to brush the barrel out. I have found that saboted bullets foul up the barrel quicker because of the amount of plastic melted into the rifling of the barrel.
As said, find what your ML likes best and stock up on it.