I'd go with the single 18" but the question is loaded. How a system sounds depends on the entire system, not just the drivers. You need to look at driver + box + room to get a good answer about how it sounds. In terms of the drivers, the single 18" will have more capability than dual 12"s, giving you more headroom and less distortion. The musicality is going to depend on the in-room response, so it will depend upon the box you build, the frequency response of the drivers in that box, and how it interacts with your room.up to 80hz, disregarding the assumption that dual subs give a "smoother" reponse (my sub location gives almost perfectly flat response at listening position).
Opinions, and reasoning behind them? Thanks!
Having 2 subs will allow you to place them at different locations in the room and smooth out the frequency response. 2 12s has almost as much cone area as a single 18, its pretty close, and the travel is about the same. Having 2 subs gives you more options. Different placement options. You can have 2 smaller enclosures, which might have higher WAF than one big one, etc.