You probably will be happy with either, I'd imagine.
When I asked about the Rythmik FV12 to the VTF-2 in terms of output, I was told that the FV12 would have more output and be closer to the bigger VTF-3, than the VTF-2. So that it is another argument for the FV12, plus like other said, it'll be more accurate because of the servo technology.
The FR (frequency response) on the Rythmik doesn't go as high as the VTF-2, which should not be a concern unless you are pairing your subwoofer with small speakers that need a very high crossover, instead of the standard (60hz/80hz). I think on the high end the FV12 rolls off at 100hz and above, but you've got big speakers, those floor-standing Polks, so you don't have to worry about the need for having a high crossover at all. (The HSU rolls off much higher, but I forget where exactly).
On the low end, the FV12, according to the spec sheet, to 20hz (if I remember right) and the VTF-2 will dig slightly deeper (in Maximum Extension mode, I think it's 18hz). Small difference, so I don't know how big of an issue it'll be and your room will dictate the actual response (may get lower). I did read from Rythmik that the FV12 does have, slightly, a more mid-bass slam as compared to other Rythmiks. This was for home theater use and he said it would appeal to the younger crowd as well. It did make me very curious though about hearing it (as I would imagine the mid-bass slam would be very nice for HT usage).
Can't go wrong with either, both are great companies. When I was looking at these two subwoofers, I was leaning toward the FV12 at the time.