Personally, I think s2x will be the longest chain, because BTC hash rate will be inadequate to keep up.
BTC hash rate responds to economics. Within the incentive structure designed by Satoshi, as tx fees pile up and compete "inadequate" hash rate is a self-correcting problem.
Users will use the chain with lower fees, where you can actually get a confirm before your beard turns white. Core will suffer a massive difficulty bomb, and if 80% of the hash is on s2x, it will take 4x longer to get a confirm. Also, there will be more xns in an s2x block, so miners are incentivized by the total fees, even when fees are only ~50% of the fees on core.
I don't really care which one wins, but I am taking a flyer on s2x for these reasons. Not in 250 BTC size, however. I am just buying the b2x futures. There I can also daytrade a bit to lower my cost basis, which I cannot do with Back's contract.