I have one, an 06 GT VERT. I also owned an 05 Vert v/6. I am very impressed with the handling of my car. Ford did a very good job with how they built the new Mustang convertibles. I have no problems cornering, I suspect no more problems than a coupe would have. I did drive a coupe GT as well. I did not notice a diference in driving either of the two.
Both the 05-06 coupes and Verts have great acceleration and it is a known fact they are not known for their great handling ability. My recommendation is to get either of the two and buy a lowering kit for better handling. Good luck on your decision. I know its a hard one to make!