One S or Galaxy Nexus?

Nexus. Nothing beats Vanilla Android and the ability to easily root and flash ROMs on your phone. You are no longer at the mercy of the carriers to depend on them to update your phone. Developer support is tremendous for the Nexus

 

S3 is probably still a few months away, you realize?

 

I just picked up a One S, it's a very solid phone. Haven't tested every little thing,but no issues so far.

 

I know May 3rd isn't that far away...heck, 2 days. But more than likely what you'll find out on May 3rd is that the Galaxy SIII is launching internationally in a few weeks and in the US later this year. Later this year means fall/winter. I applaud you for waiting till the 3rd to find out more info and being an informed consumer...but I think you'll find out you're in for quite a wait for the SIII.

Edit: Galaxy Nexus or One S are both great phones. It just depends on what you'll be doing with them as to which I would recommend. Power users that root and run custom roms love the Nexus. If you just want a nice phone with a beautiful UI and you're not planning on modding, the One S would probably be the way to go.