Hi Mark -
I'd say the Triumph is the better choice. It's aerodynamically cleaner to begin with, and probably a better platform for future mods.
The Midget's windshield looks to be nearly flat, which makes it pretty much irrelevant what kind of roof follows, since the flow off the top & sides of the windscreen will be highly turbulent anyway. The trailing wake coming off this car is pretty much going to be the size of the projected frontal area.
As for the Triumph, if I had to guess (always dangerous in aero) I'd say the rear hatch angle is too steep to permit clean flow off the roof & sides all the way to the tail end. But the opportunity is there to add a spoiler to clean things up, along the lines of Ernie Roger's VW New Beetle:
http://max-mpg.com/html/tech/main.htm
cheers-
Darin