Well I guess it really comes down to what you're comfortable with spending. My theater is basically the same size as yours and I have dual XS30's and I think they are great value, but of course what I consider value might be totally different from what you call value. IMO yes the XS30 is worth the extra money after talking with Tom for a week or two about the two subs.
One XS30 is better then a single XV15's but dual XV15's would be better then a single XS30, but if you had the budget dual XS30's would defiantly be a better setup in value.
JTR Noesis 228HT (LCR)
Axiom Audio QS8 surrounds
Sherbourn PA 7-350
Falcon Screens FVHD105
Dual PSA XS30's (gone but not forgotten)