At the moment it's a toss up between the Samsung Galaxy S3 and the HTC One X.
I've read comparison reviews and watched a few videos that showed them both in use. One thing that stood out for me in what I've seen are some reviewers saying that the One X sometime lags a bit or pauses, something the S3 doesn't do. I also saw a video where the guy was pinch zooming in on a web page and the S3 worked as I would expect but the One X seemed to zoom in and then reflow the text which looked a bit odd. Also the One X's on-screen keyboard didn't look quite as good as the S3's.
I know that there's an update available (or soon to be available) for the One X, I'm wondering what things this fixes.
I'm on Vodafone, if that matters. I'm not terribly interested in music or watching videos on my phone, a good camera would be nice but not essential. More important I get a phone with minimal or no lag as I flick between and use apps (my old HTC Magic can take 30 seconds to go back to the home screen and redraw it!) I would prefer a stock Android but I could live with TouchWiz or Sense I guess.
Also I'm a fairly heavy user of Google apps (Gmail, Maps, etc) but I assume that this isn't a problem for either phone. And I guess GPS works ok on both (vaguely remember some Samsung phones having broken GPS)?