so funny... i am in the same boat.
picked up the one s for free on Sat. (3 days ago now) and I am having a hard time finding any MAJOR flaws. I have had android almost since the beginning (had a hero on sprint) and this phone is absolutely amazing. However, i thought for sure i would be taking it back for an s3, but i just dont see a big enough difference in the things that will really matter to make the $230 difference (i got mine for free and tmo just announced value plan pricing for the s3 will be $229)!
Almost any "problem" i have can be fixed with root and a new rom. As far as the new sense goes, if you havent tried it.. you need to. I have had a few sense phones including the Evo, and this version is done VERY nice!! In the past i have almost always rooted and had some version of stock android, but with sense 4 i dont feel as compelled to go stock. Its done very nice (although i still switched the launcher to Nova cause i didnt like the 4x4 icon grid of the sense launcher on such a large screen... needs to be 6x4 IMO).
Battery life is outstanding (consistently getting a day+ with HEAVY use) however, you can get better with root. and HTC will unlock the bootloader for you!
I am really trying hard to find a real reason to get the s3... but i am not finding one that is big enough to make sense (pun intended).
I am still gonna wait to pass judgement until i get to mess with an s3, but its going to have to be something really special to make me switch.
my 2 cents. Good luck with your decision!!