yeah you guys are right, I feel like a broken record a lot of the time....I just have yet to hear or hear of anything better (except for the w7, but that's in a price range all it's own), so until then.....
If I was a mod believe me I would make a sticky, it would be a good idea, but I don't really have the time to type up a huge FAQ just to have it fall back in the thread archive like all the others.
As for 2 12s vs 1 15, in general the 15 will be able to play lower due to a lower resonant frequency, but the pair of 12's 9 times out of 10 will out displace the 15, meaning they will have the ability to get slightly louder at full output. However, to reach full output, they'll need twice as much power and a larger box (if that's a concern, which it appears it's not). Of course your budget would allow for a pair of 15s if you so desired......and if you could fit a 6cf ported box in your trunk.
[warning, incoming broken record]
It seems like a pair of XXX 15's would work if you could fit them, if not then a single 15 or a pair of 12s. Based on your post I would port, and tune to around 30hz if not slightly lower, if you got the pair you would want somewhere around 2000rms. You have the budget to allow for a very nice amp, a US Amps USA-2000 seems to fit the bill perfectly, and that's an extremely high quality amp. If you went with a single 15 instead, somewhere around 1500rms at 1 or 4ohm would be best, there are quite a few choices from companies like US Amps, Cadence, or even DLS belive it or not.