Neither. I'd opt for a S2 today and later, another S2.
Unfortunately, one needs almost nine XS30s to equal the out of a single OS, In my opinion, the flaw in the OS is, it doesn't dig deep enough and one loses out on the lower octave.
(if I'm wrong on this point, please, someone, say so)
OTOH, the S2 digs deep, gives one the lower octave and does so with authority. Looking at the lower, 10Hz octave, a single S2 equals four XS30s. In my opinion, if the budget is there, a single S2 is the best price/performance with an eye on getting a second one as soon as possible.
At this price point, another choice to seriously consider, is the PSA Triax as that's an output monster that digs deeper than Hades and with a pair, one can easily expect to get single digit performance at reference level output.. Two of those babies and quite literally, you're going need earthquake insurance. Check out the last couple of pages in the Triax thread.
In our case, our budget would only support two FV15HPs and later this year, I expect to add a third to complete the hat trick. In my opinion, the FV15HP was the middle ground between five XS30s and/or two S2s. The compromise of the FV15HP was, price/performance vs either the XS30 and the S2.
In our case, at 16Hz, the choice was, five XS30s or two FV15HPs. You do the math as to price/performance. Again, I would rather have two S2s or two Triax subs but we can only afford two FV15HPs. Our choice was based on financial compromise. The above is a brief summation of my struggle with the question you're asking about, what my final choice was and why.
In a perfect world, Donald Trump would buy me three S2s or three Triax subs and tell me to be happy and fer-git-about-it.