Personal preference really, if I hadn't ordered all the accessories for the S3 already I'd be more inclined to have a One X as a daily because of the screen. I'm still undecided on the screen of the S3, it's still a bloody good screen compared to what else is available but the difference is more noticeable now I've had them side by side. It's a hard choice, they're both very nice phones that are ridiculously fast.
The S3 also comes with more features baked into TouchWiz than Sense on the One X.
Have a good play with both and if possible on a friends phone away from the store sales people who try rush you into a decision, Voda give you 7 days to change your phone if you're not happy with it btw