It's not necessary that more expensive 15's will need bigger boxes or need more power etc.

I find that the more expensive drivers you go they prefer smaller boxes, and also are very efficient.
Particular SQ subs, which are usually expensive- $500+ for a good one.
They don't need heaps of power, usually require smaller boxes- but then again this depends on the T/S specs.

SPL subs, however which again are expensive, do require heaps of power, but with the more power u have the louder they go/shine.

IMO any sub thats in the $100-$300 range, are pretty much all the same, once you spend more you can really appreciate the quality subs.

And about SVC and DVC they don't handle the temp ratings any better.

A Dual voicecoil is essentially a SVC split in two. DVC is good because you can have more flexibility. ie. dual 2ohm- series for 4ohm load to say a 4channel bridged amp, or parrallel-1ohm for a high powered monoblock.

There are more factors in Heat cooling, such as basket design.

Again 12's are great all rounders, enough surface area, don't require large boxes/save space.

If you can fit a larger diameter subwoofer, DO IT, but only when you have maxed out on a quality sub.

ie/ no point going a 15inch Alpine Type-R, when you can get an Alpine Type-X 12 instead for the same price, which would have better SQ, require a smaller box etc.

Also IMO- spend as much money as you can on your front speakers, they are the most important. $1000 sub and $200 splits ain't gonna cut it.