WHICH ONE TO BUY? A BIT INFORMATION FOR U Xperia X10 or iphone 4
 SONY ERRICSON
SONY ERRICSON The Sony Ericsson Xperia X10 is a
 high-end smartphone designed by Sony Ericsson in the Xperia series.Its running with ANDROID OPERATING SYSTEM.
ANDEROID OS exactly is the GOOGLES OS for mobile device with ARM processors are used extensively in consumer electronics, including PDAs, mobile phones, digital media and music players, hand-held game consoles, calculators and computer peripherals such as hard drives and routers. 
Screen       : 4.0 inch touch screen, Hardware 16M Colors, 65,356-colour.
              (480 x 854 pixels) FWVGA TFT 
Camera       : 8.1 MP with Auto focus, Face recognition,
               Geo-tagging, Image and video stabilizer, Smile detection
               Video WVGA, 
OS           : Android 1.6, soon with Androind Eclair 2.1 
Input Touchscreen, Accelerometer, Digital Compass, Proximity and ambient light sensors, Headset controls 
CPU          : 1 GHz Qualcomm Snapdragon 
Memory       : 384 MB 
Storage      : 1 GB NAND Flash in phone, up to 16 GB on microSDHC memory card 
Battery      : Rechargeable and replacable, Li-Po 1500 mAh (BST-41).    
Physical size  119 x 63 x 13 mm 
Weight       : 135 g with battery 
 IPHONE 4G
IPHONE 4GThe most noticeable difference between the iPhone 4 and previous generations is a new design. The primary new design feature is a stainless steel frame. Its 3.5 inches (89 mm) LED backlit liquid crystal display with an increased 960-by-640 pixel resolution is marketed as the "Retina Display.
Developer           :  Apple Inc. 
Manufacturer        : Foxconn (OEM)[1] 
Type                : Slate smartphone 
Release date June 24, 2010 (2010-06-24)[a]  
Operating system iOS 4.0Released June 21, 2010; 21 days ago (2010-06-21) 
Power               : Built-in rechargeable li-ion battery
                      3.7 V 1420 mAh[3] 
CPU                 : Apple A4 (ARM Cortex-A8)[4] 
Storage             : capacity 16 GB or 32 GB flash memory 
Memory              : 512 MB eDRAM[5]
Display             : LED backlit IPS LCD
                                       3.5-inch screen (diagonally)
                                       640-by-960-pixel resolution at 326 ppi
                                       800:1 contrast ratio 
Input Multi-touch touchscreen display
Push-buttons
Microphone                           :Two microphones
3-axis gyroscope
3-axis accelerometer
Digital compass
Proximity sensor
Ambient light sensor 
Camera Rear5 MP
HD video (720p) at 30 fps
1.75 μm size pixels
5× digital zoom
LED flash
Front
0.3 MP (VGA)
SD video (480p) at 30 fps
25.17 µs size pixels 
Connectivity Quad band
UMTS/HSDPA/HSUPA
(850, 900, 1900, 2100 MHz)
GSM/EDGE
(850, 900, 1800, 1900 MHz)
Wi-Fi (802.11b/g/n) (2.4 GHz only)
Bluetooth 2.1 + EDR 
Online services App Store, iBook Store, iTunes Store, MobileMe 
Dimensions 115.2 mm (4.54 in) (h)
58.66 mm (2.309 in) (w)
9.3 mm (0.37 in) (d) 
Weight 137 g (4.8 oz) additional info for IPHONE 4G  :Technical specifications Apple Iphone 4g
The iPhone 4G will be 10% thinner with a similar aesthetic look as the 3GS
Improved battery life over its predecessor instead of diminished battery life in the case of the 3GS
Revamped iPhone OS
AMOLED Display
Improved Push support
Flash, if Adobe can develop it soon enough
Tougher anti-jailbreaking measures