Reply to 1 15" Sub or 2 12" subs?

Hi all,

I am looking at getting 1/2 subs to compliment my Wharfedale Titan 15a Powered Speakers. I am a mobile DJ, and have had these speakers for about a year now. They put out a decent amount of bass for a small room, but I am starting to get some bookings for some bigger rooms with more people and believe I will probably need a sub to get the volume that I will need. I don't need a massive amount of bass as I'm not really going to be playing any really bass heavy music, just normal party/wedding music.

So my question is, should I go with 1 15" sub (http://www.andertons.co.uk/subwoofer...in-in-grey.asp) OR 2 12" subs (http://www.andertons.co.uk/subwoofer...bass-bin.asp)?

My head is telling me to just get 1 15" (as it would work out a lot cheaper) but I believe that having 2 subs will look a lot neater (1 underneath each powered top). If I decided to only get 1 15", where would be the best place to position it? In the middle of the 2 top speakers?

Or should I just get 1 15" for now and then save up for another one after?