The Rythmik L12 and F12 have these things in common: They were both designed by Brian Ding, owner of Rythmik, a very knowledgeable and talented designer. They both have 12" drivers. And they both have the Rythmik Direct Servo-Feedback circuit built into their plate amps. But the amp itself is different in the two, as are the drivers, whether aluminum or paper coned (aluminum in the Rythmik F12, paper in the F12G, the G designating GR Research, whose Danny Richie worked on the development of the driver with Mr. Ding, as well as on the State-Of-The-Art GR Research/Rythmik Open Baffle/Dipole Direct Servo-Feedback Subwoofer, the only sub of it’s type in the world, and the best. Except for the Eminent Technology Rotary Subwoofer, of course!). And the enclosures are different. They both share the Rythmik "house sound"---stopping on a dime, as they say. No bloat and boom, lean and clean. The controls (PEQ, x/o, phase, damping) on the F12 are more extensive as well.
The choice is, as always, a matter of how much you have and are willing to spend. Jim Salk installs the F series Rythmik subs in his bigger speakers, and builds custom enclosures for both the F12 and F15.