I am a Martin Logan owner and have been quite pleased. In terms of output, the MFW-15 will offer more output. The Abyss is a nice subwoofer. I am a much bigger fan of their ESL Series subs (Grotto, Depth, Descent)
You might be able to find a used Grotto for close to the same price as the ones above. The ESL Series are servo controlled and are in another league than the Dynamo and Abyss. The Abyss does offer the ability to configure it to fire forward or down which can help with certain setups.
The MFW-15 will undeniably offer more bang for the buck. It looks like a 1000 Dollar+ Sub. I would recommend checking out the pros and cons concerning some people having issues with amplifiers. However, it seems that the new amplifier has helped greatly in reducing failures. These issues are very contentious and have caused many arguments and locked/deleted threads.
I will only say that there have been exceedingly few people needing repair with Martin Logan subwoofers. Also, the Abyss is a sealed enclosure and is excellent with music as well as HT.