You are right the GPS issue is not just your problem. However, there is a quick fix already posted even before you posted that it was a "problem" for you. Just take the 5 minutes to fix it and I wouldn't complain. What 5 minute fix does the Iphone 4 antenna issue have? (OHHH - cheap shot by me)Link below for the GPS fix:
http://www.androidcentral.com/quick-fix-gps-issues-your-new-samsung-galaxy-s-series-phone
Lag -- I don't know what you are talking about. It is SUPER fast.
Screen -- Iphone 4 pixels are better ... you can't see them as advertised ... galaxy S you can (if you have your eye an inch away from the screen -- I don't use it like that) I personally think the colors and vibrancy of the galaxy is better (maybe preference) ... and it is BIGGER (but not too big - Droid X).
3G -- ok, valid ... but I don't think I need that feature ... plus, I am sure this is not the "phone" so a future update or Rom might take care of it ... keep in mind this is potentially a two year purchase.
Battery -- ok ... but I think you are comparing an older Iphone to it ... and keep in mind the bigger screen. Galaxy S probably has shorter battery time (too lazy and don't care to do a huge test) ... but I think the Galaxy S battery is sufficient. I have not run out yet (in a day -- charge it every night by my bed).
My gripes ... I don't like the capacitive buttons (yet at least). I am also a former Blackberry user. I don't like how the lights of the buttons go away so fast ... and often I have to push twice, or I accidentaly push the wrong button, or there is lag (the ONLY place I have seen lag) ... maybe just make the screen bigger and make these fixed buttons on the screen (I never have problems with any lag on the buttons on the screen). The light issue should be an easy fix with a software update (I hope!) - maybe responsiveness too.
Also a technical gripe ... I can't get "PC Internet" to work when the phone is connected to WiFi -- oh YEAH ... this is a comparison to Iphone ... which can't do tether out of the box anyways ... nevermind (still a gripe though -- SOFTWARE UPDATE for that please!)
BIG Gripe -- the Samsung computer software for the Galaxy (Kies) ABSOLUTELY SUCKS!!!!!!!!!!!!!!!!!!!!!!! There are HUGE BUGS for it, especially with Windows 7. VERY unprofessional software. That being said -- keep in mind that you don't HAVE to have it ... the phone still connects and I have been able to do everything without the software ... but it definately has some huge issues to resolve. This is by far the BIGGEST and most RIDICULOUS problem ... but not the one that I would change first ... I would change the capacitive buttons first because this causes me the most real trouble on the most frequent basis (not THAT often).
Something else different (not a gripe) ... but the WEIGHT! SO DIFFERENT! The Iphone 4 feels like it is solid metal because it is SO HEAVY ... WHAT did they PUT IN THERE! Up to your preference -- to me, Iphone 4 (weight wise only considered) feels like a sexy tank while the galaxy feels like a sexy "carbon fiber" race-car -- it is MUCH lighter. I thought the battery was missing or something.
ALL of that being said -- I am definately a Galaxy S guy.
GPS - I've tried that fix multiple times along with a dozen or so other things I've seen on this and other forums. That fix doesn't really work. It may make a slight improvement, but using the wireless networks only to pinpoint a location a half mile off isn't GPS. Plenty of other people are saying the same thing.
Battery - this thread was a comparison. When comparing the two, the battery on the Captivate is worse.
3G - you may not think it's an issue for you, but it's not just a battery thing. I've been places where a call couldn't be made using 3G - even though 3g was available there - likely do to network overuse. Flip the phone to Edge - no problem making a call. It's a bigger issue than you realize.