There are lots of differences. In short, NXT is much better.
- building parts:
bricks, plates, beams, studless beams studless beams only
2 geared DC motors 3 geared DC motors with integrated encoders
2 Touch, 1 Light sensor 1 Touch, 1 Light, 1 Sound, 1 Ultrasonic sensor
- intelligent brick:
RCX, 8 bit microcontroller NXT, 32 bit microcontroller
small, 4 digit display big graphic display
There are other differences, but don't remember them all by heart. Do a search on Google.