Specifications of Samsung Galaxy S2:
The Dimensions of Samsung Galaxy S2 is 125.3 x 66.09 x 8.5 mm.
The Weight of Samsung Galaxy S2 is 116 g.
The Display Size of Samsung Galaxy S2 is 4.3 inches, 480 x 800 pixels.
The Memory Internal of Samsung Galaxy S2 is 16 GB inbuilt, 1 GB RAM.
The Bluetooth of Samsung Galaxy S2 is v3.0The Camera of Samsung Galaxy S2 is 8 MP with autofocus.
The Features of Samsung Galaxy S2 is Face detection, Smile detection, Geo tagging, Touch to focus.
The Sensors of Samsung Galaxy S2 is Accelerometer, Gyroscope, Compass.
The Standby time of Samsung Galaxy S2 is 710 hours, 610 hours on 3G.
The Talk time of Samsung Galaxy S2 is 1100 minutes, 520 minutes on 3G
Specifications of HTC Sensation:
The Dimensions of 126.1 x 65.4 x 11.3 mm (4.96 x 2.57 x 0.44 in)
The Weight of 148 g (5.22 oz)
The Display Size of 540 x 960 pixels, 4.3 inches (~256 ppi pixel density)
The Memory Internal of 1 GB storage, 768 MB RAM
The Bluetooth of v3.0 with A2DP
The Features of Android OS, v2.3.4 (Gingerbread), upgradable to v4.0 (Ice Cream Sandwich)
The Sensors of Accelerometer, gyro, proximity, compass
The Standby time ofUp to 350 h (2G) / Up to 400 h (3G)
The Talk time of Up to 8 h 20 min (2G) / Up to 6 h 40 min (3G)
By comparing the specification between these two my opinion is Samsung Galaxy S2. The price of unlocked HTC phone is $569.99 and the locked phone is $246.75. The price of unlocked Samsung mobile is $329.99 and the locked mobile is $186.87. So my suggestion for you is locked phone is best to buy in order to save money. Then get it unlocked using unlock code at http://www.superunlockcodes.com/samsung-i9100-galaxy-s-ii/rs12wp9/
,and unlock it easily.