Focal ratio controls the concentration of light that an instrument puts on the film or eye doing the observation. In still photography it's called an f-stop or f-ratio. f6 really means f-ratio = 1/6 and f4.5 really means f-ratio = 1/4.5, so f4.5 is "brighter." For stars, though, if your 'scope is focused, the images are points which are fully concentrated anyway. The brightness of stars, double stars, clusters etc., is only controlled by the size of your objective mirror (i.e how much light gets into your telescope). For nebulae, f-ratio does make a difference, just as it does for photography. The ratio of brightness important for photography would be (6/4.5)^2 which is about 2, but for naked eye we do a bunch of things to change that. Eyepieces spread the image out (which is what really happens in order to magnify it) and that decreases the brightness. But lets say you want to look at the Orion nebula at 40 power (which is a good thing to do). An f4.5 will show more nebula than an f6, definitely.

The usual drawbacks to better (lower) f-ratios is cost. (You know this if you've ever compared the cost of an f1.4 camera lense with an f3.5 lens.)

The only observational problem would be that the Moon would be EVEN brighter than it already is. To do a lot of moon observation, particularly near full moon, you really need some kind of filter to cut the light down, otherwise the glare will prevent you from seeing much.

If your kit includes a mirror blank and you intend to grind and polish the curve yourself, f4.5 will be more work since this requires a deeper curve (more work total plus it is harder to polish a mirror with higher curvature.

Gregg