Aloha Dave,
I have a TF and a TR3A and will second the comments from Sam and George. The TF is basically a TD with exterior modifications to update its appearance. My TF has a very low first gear, in a pinch on a flat you can start from a stop in second if you can't find first gear. Cruising at about 55 MPH is running at about 4000 RPM. The drum brakes are not nearly as good as the TR3's front disk/rear drum set up. I'm 5 ft 6 and don't have a leg room problem, however with the hood up I don't have much head room to spare. Also with the hood down, the top of your head will probably be above the top of the wind screen. The TR3 accelerates quickly, cruises at 60 MPH at about 3000 RPM and braking is better. There is definitely more leg room, I need to stretch to depress the clutch to the floor board with the seat forward. I also think there is more head room.
The MG "T"s are not as fragile as they appear, however they are for cruising with that classic look down winding country roads. The TR3 can do the same with a more modern look and also handle highway speeds.
Safety Fast,
Dave