What do you want from a phone?
The S3 is more powerful, has a bigger, higher resolution display, more storage, expandable storage, and a higher-capacity, replaceable battery. The One S is smaller, less expensive, you may prefer the style. And then there's the question of whether you prefer HTC or Samsung's software (Sense vs Touchwiz).
But it's a matter of what you need. You may think the larger display of the S3 is a clear win, or you may think the phone is just too large. You may think that the One S's storage (16GB, I think about 10GB available for media, non-expandable) is fine for you, or it might be too small and rule the phone out completely. Or you may think that either would work for you, in which case I'd suggest having a look at both for real and deciding which you'd prefer.