If you're going to make kick panels, you should probably use a 6.5, you will get alot stronger mid range in general.
CDT, Focal, RE, Diamond, Quart, Memphis, Alumapro, Rainbow, etc.
there are literally dozens of companies that make great equipment.
to me it depends on what the purpose you are after, as some will sound better, and some will get much louder,
some will have a brighter response, while some will have a very prominant mid range /midbass.
the best way is to listen to them, although I'm personally quite drawn to the RE XXX 6.5, www.reaudio.com