Hi Stephen,
In relation to F ratio, I guess it depends on where you are sourcing your mirror. If you are planning on buying an off the shelf GSO mirror then the F ratio will be F5 for a 10 or 12 inch and F4.5 for a 16 inch. If you are getting a mirror custom made by someone like Mark Suchting, then you as the customer can specify any reasonable F ratio with an expectation of a high quality diffraction limited mirror as an outcome.
In a practical sense for visual observing, assuming high quality optics and well built scope structure, other than requiring a coma corrector somewhere below F6 and experiencing tighter focusing margins with reducing F ratio I suspect you will find the visual image quality will be about the same regardless of F ratio.
I suggest you set out some basic criteria such as maximum eyepiece height, maximum weight, any size constraints ( both packed up and assembled), minimum F ratio ( I suggest not below F4) and see where that leads you.
For the above discussion, eyepiece height is usually about the same as the primary mirror focal length.
I find it a fascinating voyage every time I build a scope, with compromise required at every turn
Cheers,