on what you're most interested in observing.

If you're mainly interested in planetary viewing the f6 [or longer] focal ratio would be the way to go [especially since as you drop below F6 aberations like coma come increasingly into the picture which, along w/ the shorter FL EPs, as Bob mentioned, f4 is probably not the way to go].

If you're interested in 'rich-field' veiwing where max true-FOV is the object then you'll get there easier w/ the f4. Another thing to keep in mind is that if you design the scope right you should be able to use a smaller secondary mirror [and therefore, reduce your obstruction] size w/ the f6

For comparison, the Mr of a 6" aperture [about 76x] is obtained w/ a 12mm EP where it takes an 8mm EP to get the same mag at f4 but on the low-power end you'll do the same thing w/ a 29mm EP that would take more than 40mm w/ a f-ratio of 6.

In any case a f6 scope [particularly if you put a 2" focuser on it, which i would do if you're interested in DSOs] would probably give you a good compromise between mag power and true field
capabilities.