LG Nexus 4 was just confirmed by LG. It's equivalent to the Optimus G. 2x-3x CPU, 2x GPU, 2x RAM over S3. Has a better aspect ratio for screen area (1280 x 768 vs 1280 x 720 res). Are you sure you want to go for the (somewhat aging) flagships of 2012?
As for the 2 phones, HTC One X has a slightly faster CPU in benchmarks, apparent after the 4.0.4 ICS update for the One X (before that it used to be edged out mostly by the S3). GPU on the S3 is way faster (50% faster), though the Tegra 3 GPU is nothing to sniff at (it's slightly faster in games than the Mali-400MP @267MHz as in the Galaxy S2 i9100 / Galaxy Note).
S3's AMOLED is 1280 x 720
PenTile subpixel arrangement (which means a slight loss of sharpness and [mildly apparent] dottiness if you look pretty close to the screen. It has darkest blacks (compared to LCD). HTC One X's S-LCD2 has same resolution, better (more natural) colors, and RGB subpixel arrangement (a full array of subpixels).
Sense is nice. More refined, heavier (maybe clunkier), and versatile (or so I know). TouchWiz is simplistic but MAY get boring after a while.
S3's Jelly Bean update has already been seeding for a good while now, while One X Jelly Bean has probably just started (or just leaks).
Battery life on the
Galaxy S3 is a good deal better than on the
One X. Jelly Bean has
boosted battery life on the Galaxy S3. This remains unconfirmed as of now for the HTC One X.
Many people tend to be paranoid about the PenTile subpixel arrangement. I notice it (GNote), but only in red colors and with some VERY small text. It's a non-issue mostly. PenTile was better implemented on S3 as well, so don't let that make a big impact on your decision. The Galaxy S1 had it, the Galaxy Nexus had it, the Galaxy Note had it, and the Galaxy S3 has it. Do note that the Galaxy Note II has RGB subpixel arrangement (this probably marks the end of PenTile era on AMOLEDs, as with the Galaxy S4, there's a VERY high probability that they'll just be able to fit 1280 x 720 RGB in, except if they decide to go 1080p, which I'm sure will lessen the PenTile dottiness even further (to the point of being negligible).
My advice, however, is to look out for the incoming phones (releasing pretty soon), as they are wayyy ahead than these flagships in terms of raw power. You also get a Nexus phone, which gets the latest updates straight from Google, without any region-specific headaches like waiting for your region update, having to live with not always getting newest updates after new flagships get released, waiting for delayed updated.
This post was a brainfart. :P
