Most Of the time I'd recommend the galaxy, I even wrote a hub on this, however there are a few specific points I'd mention with regards to the sgs. I don't know about brand new versions of the sgs but a lot of people have trouble connecting it to their pc. If you sync a lot or need constant data transfer from a pc or need to sync to outlook you may be taking a bit of a gamble.
The iphone will work right out the box and stay the way you bought it til it dies, it's good, there are a few flaws but on the whole the user experience is slick.
On the other hand if you may watch a few movies, don't want to pay premiums for the same hardware and basic usage as everybody else... Want a little more flexibility and don't want Jobs telling you how to wipe go sgs.