I would do ported but thats just me. It would be a little cheaper and im no expert but would imagine the output would be similar. Plus if u wanted it to = the same amount of money as the 15's you could upgrade and get the cooling option on the 12's.
i say 15s sealed, those q's seem to perform very well in sealed enclosures and im sure you will have a much more musical sounding sub stage compared to the 12s ported, probably the same loudness as well, either one could be louder i would guess really depends on vehicle and space.. ect
going sealed in the biggest recommended box should give you that deep bass, those subs are meant to be low end monsters! so having great output down to 30 hz should not be a problem with a big sealed box and 15" drivers you will have more low end extension than that, but music really just doesnt get that low for the most part. what vehicle would this set up be going into?
if you want to wait a week i can let you hear my Q. but i would say dual 15"s with low qti or what ever it is and extra spider and maybe cooling how much power are you going to run?