How about using actual numbers? They're easy to find by simply looking at the SSA Product page for the XCon. The T/S spec "Sd" is effective cone area stated in cm^2. If you want it in inches, just convert. That gives you 480 cm^2 per 12 which is 74.4 in^2. The 15s are 810 cm^2 or 125.55 in^2. That makes 5 12s roughly equivalent to 3 15s. Add another of each and the percentage cone area advantage is pretty significant for the 15s at over 10%. However 10% isn't an audible difference. The 15s also want a slightly smaller enclosure. All that said, in a high power application the increase in heat dissipation allowed by the extra 2 motors of the 12s may reduce power compression and more than make up for the cone area difference. In a daily system, you aren't going to notice the difference one way or the other. If it were me, I'd go with the 15s for the cost savings and space savings.