Here's the diff:
Sensor:
a550 - Exmor CMOS, 14.2mp
a500 - Exmor CMOS, 12.4mp
a380 - CCD, 14.2mp
a330 - CCD, 10.1mp
Grip:
a5xx - manly grip
a3xx - ladies' grip (no offence to the guys who bought it..all bery manly k ;p )
Max Shooting speed:
a550 - 7fps
a500 - 5fps
a3xx - 2.5fps
LCD (tilt measured from upright position |/ ):
a550 - 3" 921,600 pixel, 90degree tilt
a500 - 3" 230,400pixel, 90degree tilt
a3xx - 2.7" 230,400pixel, 65degree tilt
Battery Charge:
a5xx - 1600mAh - srp $110
a3xx - 900mAh - srp $129 (using the small Handycam battery)
Those above, + other features like MF check live view (bery kool), higher ISO will be the main difference.
The general consensus is that becasue a5xx has a better sensor, the overall result is better as well (noise control, colour, etc etc)
If budget is an issue, just stick to A330.
If can up a bit, prefer larger grip with a better sensor, get a550.
If you like a5xx, don't need 14.2mp and dun need a LCD that makes you drool, save the $100 and get the a500.
Hopefully this super-compressed-yet-detailed comparism is useful :sweatsm: