I completely empathise with you FK1983. However, I agree that the iPhone is a more polished piece of kit with their AppStore. However, the SGS you have must surely be defective. Mine runs like greased lightning at the moment, not noticed any lag in any applications. The GPS appears to work well, usually +/- 5m accuracy when in direct view or in a pocket. I also do not lose any reception when linking my fingers around the perimeter of the phone as you do with the iPhone 4. If you have the option I would exchange your SGS for another one and take that for a test drive, if it doesn't work out for you, revert to Apple.
If you have a properly functioning SGS, I think you wil be suitably impressed over the 3GS, however at the moment the 'Android marketplace' is behind the 'Apple App Store'. There are no where near as many hardcore gaming developerrs in the Android marketplace for now but AFAIK this will change with the number of sales of Android based OS phones being sold now.
Don't get me wrong though, I'm not totally convinced on the SGS. The Kies software is a joke The Technical Support takes forever to call back and then appears to be a non-specific agent who you have to talk them around the phone, rather than receiving expert advice. The phones themselves should never have had this many bad batches passing through quality checks and I think that Samsung should definitely have a support website with all the talk throughs for various settings, with video clips rather than us all having to blindly find workarounds.
Without this site the phone would have gone back within 48 hours, so I share your frustrations but now that it's firing on all cylinders (minus Kies) it's a very tidy piece of kit. To answer the OP, for a 16 year old it depends whether you like to follow the crowd with the protection that can offer along with an initially easier setup with Apple or whether you prefer to have something that costs less initially, requires some concentration and frustration initially to find the correct menus and setting but overall has excellent potential, especially with Froyo and it's associated benefits due supposedly soon.
What I would suggest is buying from a retailer that will allow you to swap the phone if you are not 100% happy with it within the first 7 days or so. This way, you can see for yourself whether it's for you.