Both.

I bought two Mindstorms several years ago. One for each of my kids that were interested.

I recently bought the RoboSapien V2, and have been leant the Lego IR tower for experimenting with broadcasting PC control to the robot. 

I think the MovIt line of hobbiest robots were more instructive in mechanics and  programming than the MindStorm set at the time. I have a WAO that has a 4 bit processor and uses bumpers to find its way around. You can program it to be a wall hugging maze solver, or reconfigure it to detect and avoid table edges.

ScottE -- Member (always) & Moderator (when needed)

I like them both.  One could actually combine the two without actually interfacing them.   The NXT could be hung on his back to send back data to your PDA or computer!  Both the RS's and Mindstorms have a lot of potential limited only by skill and imagination.