I'd go with Vex over NXT any day.
Vex: 16 analog/digital/IO ports; 8 PWM motor ports; 6 Interrupt Ports
NXT: 4 input ports; 3 motor ports
Vex has so many more motor and sensor ports available, and it provides a great platform for future projects (like building a Bluetooth module or serving as an FRC co-processor). Vex is also better suited for FRC-type robots, while NXT is more suited for making toy-like robots.