On paper, the F12 could sound better because of its servo mechanism and likely higher damping, while the legend, with double the surface area, would likely have more headroom.
But in a room, I think for the price of one F12 you could get two legends. The room will always be your #1 and #2 and #3 bottleneck to bass sound quality (below ~400hz or so)
If you really want the best possible bass, you need to invest in multiple subs (2, 3, or 4), measurement mic/preamp to find optimal placement, and electronics to integrate their levels/time delays.
I think dual Legend should be a great choice. I would expect it to outdo the 12" Rythmik in either output or extension. For a sealed sub, I would be looking at the F15, not the F12, from rythmik. Again, while subs can have small contributions to perceived "sound quality", your in-room frequency response is generally the number one determinant.
I don't know much about Rythmik, but from what I've read, and seen on their website they make fine subs.
I have dual Legends, and they are IMO the best sounding subs I've had. And I have had a lot of subs..
HSU VTF/3/3, VTF2/3, 2 VTF1's, Rel T1, MBM-12, Velodyne ULS-12 (very dated), and two sealed
Dayton 10" Titanic subs, and a top of the line Mirage (forgot model no.). So it's not like the Legends
are my first "real subwoofers".
That said I highly recommend the Epik's.
Maybe somebody can chime in with their experience with Rythmik.