First: YES, bigger is ALWAYS better. Even if you don't want negative effects like too low of a Q factor from the design. Why? You can do what DC does, HUGE motor, loose gap to coil. It's actually pretty hard to stay on the the low Q side when you are designing and building subs. You can do anything with a large motor, but not much at all with a smaller motor.
Larger motors are best utilized for power handling. Some don't realize that larger motors have MUCH less power compression(coil power handling via the motors say).
If you are wanting a long excursion subwoofer YES.
Remember this: If you want a low FS AND High power handling then you need a large motor for this reason alone because you will need a higher movingmass AND stiffer suspension which both work the opposite at the sub's FS. The heavy cone/coil will try and lower that FS, but the stiff spiders for the power handling are going to raise the Q a bit.
Second: If you want people thinking a smaller subwoofer is a 15" then you need a high XMAX subwoofer, thus, you need a strong motor.
1. Alot of xmax in the gap drives Q up in comparison to a shorter one due to flux density being higher with a closer gap to coil height ratio.
2. If you want it to handle a lot of power you need some stiff spiders which also raises Q.
3. You are also going to want a decently low FS however the long coil will take care of this via weight which is another thing more xmax brings to the table.
So, for a ported low end beast, use a long flat aluminum coil that fits snug in the gap and the spiders would be hard to give a specific figure for, since spider suppliers generally don't give you CMS parameters and stuff. I just go by experience.
Always try and keep the Q low....for low end NO, you dont want too light of an MMS....unless you're using a weak motor. The reason i am assuming you want low end is because you say you wan the sub to sound like a 15".
A woofer tester like the Wt2 is great too....sundownz Uses it.