It depends whether you're just talking about focal ratio, or focal length as well. The 10" SCT's for example, they are f/10 but with a focal length of 2500mm. They are double the magnification of your f/5 dob, which some would argue makes them better for planets. At f/10, they are also more forgiving on eyepiece design and are good for planetary/lunar imaging
because of that longer focal length.
The f/5 dob's are great for widefield DSO or lunar viewing, because of the shorter focal length compared to the 10" SCT's.
If you're just talking about focal ratio for teh same scope, there are arguments that masking down the scope can help in poor seeing, but you're also throwing away resolving power. The f/6 8" dob's are slightly more forgiving on collimation and eyepiece design, but not significantly and collimation is collimation - you need to learn to get it right whether you've got an f/5 or an f/6.
btw i'm impressed at your restraint! You could argue to your wife though, that you won't *use* it as such, but while waiting until the 31st July you can do a bunch of mods to the scope - flocking, fix the base motion, etc
