Get the bigger sub. I started out with 12 in and worried about adding additional subs and their size. I now have three 18 in subs. It is funny how in 3-6 months we can afford a better sub if we are not completely happy with the bass in the room or just want to upgrade. Something tells me if you really want a second sub down the road, it will happen!
I have a lot of space to pressurize and there is no way around it.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500