What are you going to use it for? If it's going to be used behind a small block in a Pinto, stick with the C-4. The C-4 is smaller, lighter, and enough people have used them for racing that performance replacement parts for the known weaknesses are readily available. Setup properly, it's hard to beat a C-4 behind a small block! The FMX was a less expensive replacement for the C-6 and, in some cases, C-4s behind 351s. It's an alright tranny, but it's heavier than the C-4 and there aren't nearly as many performance parts available.