I have the flat tail version myself, and wish I could say the diff in the 2 of them. I can say though that I have heard there's not a big diff in them.
If you do have the flat version though, I stress the need for flying wires...as that in itself is a big diff in the 2 planes.
The foiled tail has built in triangulation, and therefore has no need for the flying wires...being a standard Dtube structure it is very stiff and strong.
The flat tail on the other hand has no triangulation for rigidity, and without the flying wires has way to much flex...until the wires are added. It takes on a whole new character I'm sure. I don't really know, as I've never flown mine without them, nor would I.
What I did though was instead of using stranded/wound metal wire, I used coated kevlar,,,this also alleviated any Rf interferance possibity that might exist if metallic wire is used.
My flat tail version is very tight though in the tail structure, as most are when assembled correctly.
I don't know if this helps at all, but it's just a few thoughts I wanted to share....good luck with your plane.
I think the Midwest Extra is one of the best kits I've ever built for sure. Take care .