Of the brands you've got to pick from, I'm honestly not allthat educated... I'm prettysure that you can get a pretty beefy RF power series sub. I think I've seen similar from Pio as well.
My box is still un-finished, and yes I'm painfully lazy, but Here's a couple shots of my 12 in action, exercising that surround to it's fullest. Again, this setup pretty easily out-hits the majority of "pair of 12's" setups I've been in a situation to parking-lot-drag against, and it's not always just generic el-cheapo subs. Mostly I'll have ppl listen to it and they'll inevitably get all anxious when I go to pop my hatch, expecting to see some flashy huge pile of amps, subs, and neon. Then when they nearly passup the simple no-logo flat-carbonfiber sub sunk into the side, they ask to hear it agian. This is exactly what I wanted. Ability to crank out some SPL while still staying clean and tight when I take it off-the-boil, And also as out-of-the-way as possible so I can still use my hatch for adventure-wagon duties.
Getting a single sub that moves twice as far as a normal one winds up being the same as having 2, only you often don't need to takeup all of that extra space. In the end it's displacement that=spl, and displacement is your cone area multiplied by how far it moves. So say subx has a 1" Peak to Peak excursion, and sub Y has a 2" peak to peak. Running 2 x's would equal that one Y, only with the Y you're using up essentially half the space. Power-wise to do this, theoretically you'd use the same power for the single as you did for the pair of singles, but in reality you might end up needing a touch more.
I guess in the end it's all a matter of what you personally are shooting for. if you want nice soundquality, there's no reason to get out of hand, a single 10 is pretty common at competitions. If you want alot of boom, perhaps 2 subs are your ticket, but it's very possible for a single nicer sub to hold it's own as well. Asfor taking up the whole back of your seat, are you going to build something or just buy a pre-fabbed box? because If it were me and my goal were to takeup the full-width, have more than enough bass, and look nice, I'd slap a nice single 15 or 12 on one side, build a big full-width slot-ported box tuned to about 25hz, and build an amp-rack into it to takeup the "dead" space on the other side. But that's just me I get bored with symmetry and all of the "me too" "systems" in cars anymore.