At the risk of committing blasphemy here (I prefer Triumphs), I think the MGB is generally a more comfortable car to drive than a Spitfire. The MGB suspension (if left stock) is softer and it rides more smoothly. Plus, the MGB interior is larger with more legroom, elbow room, etc. MGs are also a bit safer if you're involved in a crash.
In general, the Spit is a sportier car. It's lighter, and a bit more nimble. Spits are also rarer than MGBs on this side of the pond, making them more of a head-turner, IMHO. Further, the Spit is an easier and less expensive car to maintain and restore, I think. Since the bonnet raises the entire front end of the car, it's supremely easy to work on with the engine and front suspension exposed.
Dollar for dollar, I think the Spitfire give you more bang for the buck. With patience and a little luck, you can get a very nicely restored Spit for less than $5,000. That's a real bargain for a sexy, European convertible sports car.
Can't do better for the money than a Spitfire! /ubbthreads/images/graemlins/driving.gif