ive actually had all three..
The MX, and XXX both need more power than 1200w to get really moving, i had a sundown 1500d and they were still both laughing at me. But dont get me wrong, they are both excellent subs. The XXX was probably the best sounding, it hit the lows with more authority, but the mag was a close second. (the XXX was also a 15") so that could explain the greater authority part.
The MX was definatley louder than the mag, but sounded no where NEAR as good. The mag is an incredible sounding woofer, Great all around woofer that can do everything. The MX, was a nice little woofer, but didnt sound AMAZING, but it sounded good, and got pretty loud.
On the other hand about the XXX, it only plays UP to 63hz or so, due to the XBL motor, so you need to have a strong set of mids. (people told me this before i got the xxx, and now i see what they are talkin about)
If you want a loud, great sounding woofer, and with that kinda power i say go for the mag hands down. May cost you a few more bucks, but in the end, you wont regret it. I wish i had mine back. lol
Good luck.