I just completed a 15/6/1, followed by a 12/6/1 Cube. I play them together in a stereo format. The 15/6/1 on the neck PU, and the 12/6/1 on the bridge PU. WOW it's a really nice setup! If I have to pick just one, it would be the 15/6/1. I have run mono through both, in my opinion the 15/6/1 is a rounder better sound that one would expect from the 15" driver. I don't think you could go wrong with either. I bought the complete kits from Leland, and man what a package he sends out. Everything was there, and it just fit PERFECT!!!! Highly recommend that option of you can.
Jon
Jon