A value decision also includes what your goal is. In both cases, because of the 6 cylinder engine, a concours restoration would not be practical due to the parts costs vs low return. Consider the condition of the body as well, the lesser of two evils is the car with the least rust and body damage. That being said, your next option would be to upgrade the drive train and undercarriage to accomodate a V8 engine. If you are looking to race, the Fastback would be a better choice than the convertible. For cruising, either would be good. There is an additional expense for convertible tops that need to be tossed into the mix. The formula is: top value minus initial cost, minus repair costs, minus fix-up (aka customizing) costs, minus the time to do the stuff.