Generally higher focal ratios reduce halo that can be seen when viewing stars and planets and are easier on affects of optical alignment problems.
A lower focal ratio shows a wider field of view and some say a sharper image as less scattered light reaches the eye. Lower focal ratio also allows for highr magnification but I'm not sure the difference between f4.5 and f6 would make that much difference to you as opposed to comparing a f10.

I've never built a telescope but I've heard it's easier to have good results from your labor with higher focal ratios and I thought Dob was pushing or recomending higher focal ratios. You might do a search on Mr Dob's telescopes for more info.