Which subs are best for home theater?

I now have a double 12" self-powered sub that I'm using in the front channel for home theater. I am thinking of adding a rear-channel subwoofer. My dealer has suggested using a single 15" down firing sub in both front and rear channels. Would a 15" in the rear be overkill? Should I use a 15" for the front? And maybe use the double 12" sub for the rear channels?

Another question would be is a double 12" sub better on audio than the single 15" sub?

Thank you in advance for your suggestions.

MR.upgrade