Hi J_rock
Can't say which gun will produce the highest quality coatings from these powders, but that both will be capable of meeting aviation standards. I would expect the F4 gun nozzle/electrode to last much longer than 7M's, but with a significant cost differential.
My experience is mainly with 7M type systems, and I would imagine that most people will have an experience bias between F4 and 7M/9M.. So I think you are in a good situation to test the many factors that are required to make a good judgement yourself. Relying on other peoples generic opinions (may not be specific to your specific materials) could be misleading.
A few other factors for consideration:
Cost and life of consumables
Deposit efficiencies
Spray rates
Power/gases use and cost
Quality and consistency
Experience and knowledge of system
It would be interesting to hear other comments. I wonder what Sulzer Metco would make of this question?