Screen Size and resolution:
The Samsung Galaxy SII has a 4.3-inch screen with 480x800 AMOLED Plus display in comparison to Apple iPhone 4's 3.5-inch 640x960 pixel display. Though the Samsung has a bigger screen with great contrast and brightness, the iPhone 4 has a higher resolution screen
Processor:
Apple's iPhone 4 sports an A4 processor believed to run at 1 Ghz, the Samsung Galaxy S II packs-in a much faster 1.2 Ghz dual-core processor
Camera:
The Apple iPhone 4 features a 5-megapixel camera that shoots 720p video and a VGA front facing camera. The Galaxy SII on the other hand shoots 8-megapixel photos and 1080p video at 30 fps. It also features a 2-megapixel front facing camera
Thickness:
The iPhone 4 is 9.3 mm thick as compared to Samsung Galaxy SII which is just 8.49mm
Back finish:
The iPhone 4 has a glass back which looks fabulous, but is a fingerprint magnet. The Samsung Galaxy SII on the other hand is a more practical plastic matte-finish
Operating system:
The iPhone 4 runs on iOS 4.3 while the Samsung Galaxy S II will run on Android 2.3 Gingerbread
Price and Plans:
iPhone 4 will launch on May 27 at Rs. 34,500 for the 16GB model and Rs. 40,900 for the 32GB model. Customers can recover the cost of the phone by subscribing to subsidized operator plans from Aircel and Airtel.
The Galaxy SII will be available from June 3 for Rs. 32,890 for the 16 GB model from Vodafone. Customers will get 1 GB data free for the first six months
Here is a quick specs comparison of the two devices