12 or 15?

A 12 will hit cleaner than a 15, but the 15 will put out higher spl and generally hit lower.  If you are a fanatic for sound quality, stick with the 12.  If you wanna rock people from blocks away, go with the 15.  Considering you are using an Eclipse sub, the sound from the 15 is still going to be very clean.  Most of the installs that I have done, where the customer listens to Rap I have suggested 15's and they have always been happy.  I've done many installs with 1 well powered 15 and in a good box that just straight up scared people.

Hmmmm....It just depended on the sub.  One was an alpine type r 15 in 1.5 cuft sealed 3/4 mdf, another was a kicker comp in 3 cuft sealed.  the last was a budget install in a dodge neon that was a fosgate "Z" in 2.25 sealed.  One of the loudest ones was 2 jl 15w6s in 1.5 cuft each sealed in a hatchback.  One of the deepest hitting setups was 2 15" audiobahn excursions in 2 cuft each running at 1 ohm off the a1200 audiobahn class d amp.  Not the loudest, but it went low and didn't get to messy doin it.  I know none of these were really SPL setups, but I've always been partial to loud AND clean.

Ever done a MQ quart system? anyone?

In my opinion 15's are ok but sometimes sound like they are dragging.  12's in the right box can really move somethings.  Also, depending on the car the size of the subs makes a difference because subs move air.  The smaller the car the less are there is to move so the smaller the sub can be.