DD-12 Vs. DD-15

I really don't know which one to get...Is it true that the DD-15 is not THX certified? I like the smaller 12" but am wondering if the 15 is a significant step up. Thoughts?
Don't worry about the THX certification it more licensing than anything. if you have a mid-large room and you are going to use this for movies.. go as big as you can afford or better yet get stereo subs which will help out with accoustics versus one big sub. Both Velo's are awesome.
I replaced my HGS-18II with the DD-15 and couldn't be any happier. The DD-15 has tighter bass, less distortion and digital room EQ. If your room is big, better go with the DD-15 or better yet, DD-18.
Music is first and foremost the most important...but I don't want it to choke up for movies.
I have the DD-15 and think it's a great deal. More than enough for my room. I called Velodyne and talked with them for a while and they said it is highly unlikly anyone would not think the 15 is not enough.
While I am sure that the DD-15 is, indeed, more than up to just about any job, I would just add that the DD-12 that I have in my system is also quite the overachiever. My room is not huge, but it does have significant bass-suck issues, and the DD-12 corrected all of those for me, and is barely coasting along (compared to its capabilities).

Either would be a good choice, I'll wager.
If music is primary - go with the 12 or a pair of 12's. It will be faster and you will not lose much, if anything for movies.
Provided both are well designed, a 12" would not be "faster" than a 15". The Velodyne DD-18 and the Bag End S18E, for example, are as fast as they need to be to keep up with any speaker, even Quads.