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Alex,
1. I would steer clear of f/4. It means it will do low power nicely but as for high power... fuggedit. Many eyepieces do not handle f/4 well, unless you plan on a box of Naglers and Pans.
f5 is closer to the optimum (which imho is f/7).
2. While I appreciate the desire for aperture, an 8" will be easily manageable for you for many years. A 10" will be a heavy lift and enough to put you off. Think about the future - we're not young anymore.
I'm also old enough (60) to appreciate this ... my 7" Skywatcher mak is easy peasy, but the 9" Santel mak is twice the weight and a real concern. While I lift it now easily enough, in 10 years time probably not. This is one reason I'm having second thoughts about selling the 7".
Another IIS member I know keeps an 8" SCT for the same reason - its not his dream scope, but it's the one he uses most because it's convenient.
IMHO it's much like cameras - once I had a big heavy DSLR thinking it was the ducks nuts, but in reality I shot 19X as many photos with my little compact that went everywhere, always.
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