The MGB is a great car, classic lines, great handling, smile on your face, joy of driving, not expensive. It's just 60s and 70s technology, and British cars do not have the reputation of being the most reliable ever made.
The Miata is a great car, perfect handling, smile on your face, joy of driving, not expensive, but the 2nd Gen came out in 1998, and is reliable. They did not sell 98s in America, so a 99 is the earliest you can buy.
With both cars, it's about feeling one with the car, and making trips to the grocery store fun.
You can't beat the 50/50 weight distribution on my 99 Miata. Steer the front with the turny wheel, steer the back with the right pedal. You have more options of where to put the car in the first few seconds you have to avoid the wreck. Plus, they're cheaper than a Honda Civic, and take very well to performance mods. There's a reason more Miatas are raced by professional drivers than any other car.
I can't tell you which car is right for you, all I would say is drive a Miata before you make your final decision.