I got to demo a ML Decent in my room for a few hours a couple of weeks ago. I wasn't that impressed. I've heard/read some very well respected people say that it is a great sub, so I think my issues boiled down to room placement and setup. But, in the few hours I was allowed to play with it, I just couldn't get it to sound right. To me, it sounded tubby, and I drove it into compression at levels that weren't much above normal listening. My DIY subs, which at that time weren't setup that all that well either (have since moved them and redid the eq completely) really embarrassed the ML. But, I guess it probably isn't fair to say that, since I already admitted I had problems locating the ML to get it to sound good.
I've listened to a DD12 at another guy's place, and will probably have him over with it this weekend, and liked it. It is a typical, small sealed sub with really slick software. Therefore, I vote for the DD15.