i have an iphone 4 and galaxy S and not the S2. All the points mentioned below are my opinion formed over 8-9 months of experience.
Pros of iphone 4
- great screen
- useful LED flash
- a little bit stable os. see below for problems faced.
- many apps.
- good camera, especially outside.
- great build quality
- good battery life.
- paid $250 for device alone. it better be good.
- resale value is good.
Cons of iphone 4
- i had to replace the phone because of bad pixel. not expected when spending $250 for device alone.
- while using it, a message came up saying the phone is overheated and needs cooling down and it shut down.
- phone hanged thrice. apple store representative told, its because of many apps running simultaneously. was shown to manually kill all apps.
- screen size very small after using 4" of GS.
- bulky compared to SGS.
- apple's restriction of using itunes, this and that.
- mine is a non-jb one. can't play videos w/o converting.
- no drag and drop for audio/video.
- BT problem. i pair it with 3-4 devices. doesn't support all. can't BT pair with my sony laptop.
- copy and paste problem.
- no widgets.
- no notification bar as in android. i know its fixed in iOS 5, but mine is still on 4.3.
- no live wallpaper. i like it.
- no true multi tasking.
- JB is a pain. Unlocking is again a pain.
- GPS doesn't lock faster.
- no tab browsing safari. i think its fixed in iOS 5.
- have faced death grip problem. at same place, i get 4 bars in SGS, i get 1-2 bar in iphone 4.
- glass back, always worried about dropping and breaking it. with a otterbox or any other case, gets super bulky.
- non-replaceable battery.
pros of SGS
- great screen. size and quality.
- seamless BT pairing with all devices.
- GPS locks early in this one.
- live wallpaper, notification bar, widgets a boon.
- awesome multi tasking. while updating apps on one side, i play games, apps get updated by downloading and installed automatically.
- tabbed browsing. option of multiple browsers.
- copy, paste not a problem.
- need no itunes. drag and drop.
- plays my 720p and 1080p videos without any conversion and jitter.
- drag and drop of files from computer.
- slim to hold, doesn't make its presence felt in pocket. iphone 4 does.
- rooting and unlocking is damn easy. Samsung has built in unlock tool. super easy.
- quite reliable till now. no problems of hanging, overheating. touchwood.
- love the built in task manager.
- got it free as a part of black friday deal. no regrets.

- user replaceable battery.
- biggest of all - memory expansion.
cons of SGS
- no LED flash. i seriously wish they provide one. will be really useful as a LED torch.
- average camera. iphone's is a little bit better. but sgs has more features.
- average battery life. iphone does 2 days easily, SGS last 1 and half day max.
- low resale value. but then i am not selling it ever.
- built in speaker not good. but still louder than iphone 4.
- samsung and at&t extremely slow at pushing OS updates.
these are some of the points. i like both devices. but somehow i prefer the SGS more. Awaiting SGS 2. i have used iphone 2G and 3GS before too. I have managed to brick my 3GS and 2G before, so i am not taking more chances with iphone 4.
Sorry for the long post.