Go to a dealer and listen to them to see which setup you like better.
"All else being equal" two subs would be louder than the exact same single sub however you are comparing two entirely different animals. There is no way to predict exact output (too many other variables) but you could use one of the programs available to get an idea of which would be louder. In any case if "money won't be an issue" this comparison is kinda like asking if you'd rather have 2 of these.....i'm plannin a project, and lets say that money wont be an issue. i'm deciding between 2 subwoofer system setups.
1) a single JL Audio 12W7 in a sealed box powered with 900-1000W RMS
2) a pair of JL 12W1v2 subs in a sealed box, powered with 600W RMS total
i'm assuming the 12W7 will have better SQ, but which setup will put out more dB's? i've seen videos of W7's pushed hard, and man, they have lots of excursion! then again, its just videos.
or will a pair of subs always put out more dB's than just one?