I own the Fisher Progressors in a 175 and I have been on the SS Magnums in a 170 (and I actually just bought a pair to replace some regular SS's that had become rock skis).
I think of these two skis as different types of skis. I think of the Progressor as a cheater GS ski, and it defintely skis like that in the two longer lengths. It is amazingly stable at speed and yet it can carve much tighter than GS turns due to the tight turn radius and the progressive sidecut (tip sidecut is a 14m radius while the tail is 17-18M). This makes the ski engage incrdeibly quick, but becuse the tail has less sidecut it is amazingly quick at exiting a carve as well and you never have th feeling that you are waiting for the ski to get through the turn the way you do with the original SS's. The Progressors are reasoably stiff which you won't notice much on groomed terrain, but off piste they can throw you some if you let your fore aft balance waver. Having said that, I really enjoy these ski in up to 8-10 inches of fresh and when on edge they are like bulldozers through crud and chop.
I think of the Magnums as slalom carvers just like the original SS's. And they defintely ski that way in the 163 and 170 lenghts. At 71mm under foot they are definitely a better ski in fresh snow, crud and chop, but when on edge they produce a tight (13.5 meters in 170) turn. I find them a little stiffer and a little burlier than the original SS's and a little better in fresh snow.
Having said all this, if you ski the SS Magnum in a 177 length and the progressor in a 170, you will have almost the same turn radius and they may feel like very similar skis.
IMO if you are looking for a slalom carver to carve tight turns all over the mounatin, buy the Magnum. If you are looking for a cheater GS ski, buy the Progressor. You can't go wrong with either ski if your expectations of what each will do well are correct. Both are stable at speed, but the progerssors are defintely more stable in GS type turns. The magnums are stiffer than the original SS's but they still are not super stable in turns wider than the ski radius.
Many people my size and weight ski the magnum in the 177 and love it. I think that the Magnum and the original SS lose their true character at the longest length (177 and 175 respectively). As well many people my height and weight ski the Porgressor in the 170 and like it and think of it more as a tight trun carver. Again, I think the ski loses some of it's true character at the shorter lengths (all relative to ones size and weight).
I hope that helps some.