I went with the 3 point brace for my convertible. It is a more difficult install, because you have to cut the plastic firewall cover to expose the third attachment point. But it will give a tighter feel to the car. Two point is better than nothing, I guess.
Bear in mind that the strut tower brace isn't a miracle cure for the convertible. Just one of several chassis reinforcements you will need. Removing the factory bolt on subframe connectors and replacing with full length weld in subframe connectors are also a must.
what are you looking for out of it? what else do you have done as far as chassis stiffening?
since i bought my vert my goal was to have it handle as best it could and be as rigid as possible. one of the the last pieces i want to install is a 3pt strut tower brace from MM, stifflers would also be a viable option as long as it could clear my 2.3 whipple