I can't speak to the MFW-15 vs. the Submersive, but I can speak to it (MFW) vs. the Rythmik D15se Servo sub. The following opinion is based on pairs of MFWs vs Rythmiks, both equalized via Audyssey. For music, the Rythmik is better (and frankly the most musical sub I've owned) but the MFW is surprisingly close given the price difference. Subjectively, I'd say for music (tightness, articulation) the MFW is 90-93% as good as the Rythmik, which says a lot. For HT I'd rate both subs as equal, but different. The Rythmik was surprisingly powerful, extended, and produced LFE with accuracy I haven't heard previously. By comparison, the MFW didn't extend quite as deep as the Rythmik but from 20 hz up delivered more impact and dynamics. I went with the MFW over the Rythmiks primarily for cosmetics and dedicated HT (my main speakers are full range and don't need sub assist) and ultimately, I think I made a good decision given my circumstances.
From what I've read of the Submersive, I think a pair of 15" Rythmiks will provide similar performance and if one accepts this notion, I'd say if one's priority is music vs. HT, go with the Submersive. If the inverse is true and your budget is tight, go with a pair (or more) of MFW-15s. In the final analysis, I think you'll be satisfied either way.