Personally, I'd get the Mustang just because of my size... I would be laughed at like no tomorrow if I got up in a Mini. My English teacher last year, who was BIG and GAY drove one... it suited him. I do however like the Mini Coopers, my girlfriend and I were looking at one for her but I don't think it's big enough to protect her with the way she drives, lawl
You're small though, 135lbs... go with the Cooper, I know they turn corners like nothing... I'd be whipping around corners all day!