The gradient is basicly a tank so it will punch it's way through just about anything but it is not as nimble as the phat(or allot of other creakers for that matter) so it tends to be better for high volume stuff. The Phat will not punch through some holes that the gradient can but it is more nimble so it is better for more technical stuff. Neither one is cutting edge but they are both good boats. I would take either one down some class IV creaks. I tend to do more low volume technical stuff and I just prefer a more nimble boat in general so given the choice of the two I would probably stick with the phat as things head into class V. If you are just planning on dropping off a bunch of clean 40'ers it realy does not matter what you get. As long as your not in a dedicated playboat (like the session +) or something like a corsica, most of the boats designed with river running/creaking in mind should work fine. If you are going to be doing stuff that is tighter, more technical, and more dangerous I would stick with a dedicated creaker. There are a bunch of creakboat designs out there that you can get fairly cheap. You might look into a topo, stuby, Y, micro, salto, big gun, or you might even find one of the newer designs pretty cheap if you look hard enough. All the creakboats out there handle very diferently, but if you can make the boat do exactly what you want it to then you realy can't go wrong with any of them.
I would say that if you are going to chose between the two of them you should go with the phat. If you are going to be going down rocky runs that probably means it will be more technical, plus given that you have tried the phat and loved it you know it will work for YOU. Try out some other creakers if you can before you buy, but if not the phat is a great boat and I think it will serve you well.