Aside from appearance, the bodies are fiberglass not steel so no rust. The brakes are generally disks not drums. Intermeccanicas can be had with rack and pinion steering, and Porsche 911 suspension for significantly better handling (not sure about others). Originals had about 90hp where any of these cars can approach 200hp.
As for appearance. If you want your car to look exactly like a Porsche, IM can accomplish that for you, from through the overrider exhaust to correct tunnel mounted heat control. However, an IM is not a Porsche, it is an IM, and a JPS is not a Porsche, it is a JPS. Each maker builds a car that is a bit different from the others. None are exactly "better" than any others. They are just different. I think, like everything else, you get what you pay for, so you need to begin by deciding what your budget is, and then, what your priorities are for spending that budget.
Also, either visit the builders, or try to find owners of their cars near you. Don't make a decision without driving the different cars! We all think our car is the best. You need to decide for yourself.