Samsung Galaxy S2

Camera.Rear-facing 8MP camera, 1080p HD video.Plenty of manual controls as in digital cameras such as adjustable ISO from 100-800 etc.

Display.Big display with Super AMOLED Plus which gives better resolution than the Samsung Galaxy S.

Wireless Connection.Wireless connection with PC which eliminates the need for USB cable.

Battery.The Samsung Galaxy S2 can last for 2 full days with average usage.

Hardware.The Samsung Galaxy S2 has a 1.2 GHz Dual-core Exynos processor, which gives faster speed than the iPhone 4S's A5 Chip.

Price.The price is comparable to iPhone 4S which available at $199, $299 and $399 with two year contract for 16GB, 32GB and 64GB models respectively.

Sources:
http://www.apple.com/
http://reviews.cnet.com/
http://www.techradar.com 
http://www.engadget.com