2 Subwoofers or 1?

I did a quick Forum search but didn't find exactly what I am looking for.  Please forgive me if this is really a duplicate question...

I've recently purchased a pair of Ariel 5Ts to go with my Audio Research amp/preamp.  I've started thinking of adding a sub woofer.  The local dealer is very high on the RELs and it seems that so are a lot or people.  Here's my question: would it be better to buy 2 Rel T/5x at 699 a piece or 1 T/9x for 1499.  The room is roughly 15 x 22 with 14 ft ceilings and open both at the back and to the right. The two smaller units are 125 watts each: the larger unit is 300 watts, if that matters. Any thoughts on the matter?

Thanks

Howard