It comes down to when do you need enrichment under load? The 7.5 will come in the earliest - when the vacuum drops to 7.5 or below, while the 5.5 will come in the latest. Nobody can tell you exactly what you need - it will depend on your specific engine, car, usage, etc..
If it was mine, I'd go with a 6.5 and see how it runs. You can always buy all 3 and then perform some tests to see what you like the best.
The most important thing is to get the proper power-valves ---> ones designed for the seat configuration of the 94 style carbs.