I think the soft vs stiff debate is irrelevant in this thread, they're both about the same as far as flex goes. The more important thing to consider is Rocker vs Camber between these two skis. Coming off the Wall, they're fairly close in stiffness to the later constructs of the Volkl Wall (the 08/09 and 09/10 versions were MUCH stiffer than their predecessors, so if you have either of those models, then this statement isn't true), but what will make a difference is the pattern in which they flex and the rocker. G-butters (to me at least) are soft in the tip and tail, stiff underfoot, however have a slightly softer flex directly underfoot (only a few cms, but I felt like they had a little play in there...the ones I tried out were by no means new, so that might play into this part). The FRs are as far as natural flex goes a bit stiffer than the G-Butters, HOWEVER the massive amount of rocker will make them feel a bit softer. So like I said, they're fairly similar as far as flex goes.
Both skis are fun, like I said the biggest difference is the Rocker vs Camber between the two, so I think that should be more of your deciding factor. Do you want to stay with a cambered park ski? Do you want to try out rocker? (I really enjoy rocker in the park, and I like the surfy edge to edge feel I get when I carve them out...but that's just me). The FRs will have a slight edge in terms of durability (in my opinion, no ski can beat them in that regard, they're nuke proof, haha), but Fat-y-pus also puts out a pretty quality product. The FR will have more snap to it, so it'll be a bit more responsive (bamboo core is amazing when it comes to responding to what you put into it). So I'd consider those things instead of flex, personally.