I've used both OAG and guide scope in my old film days. Each has it's advantages / disadvantages. Now things have changed in the 20 years I've used them so I'm not sure how relevant my comments will be in the digital age. But since no one else has commented....
OAG advantage is that it eliminates differential flexure between guide scope and main imaging scope and internal flexure of the imaging scope. It's one of the reasons I got one. I could never completely eliminate flex at long focal lengths otherwise. But then I was using "flexible" scopes.
Disadvantage used to be the limited number of guide stars to choose from. I don't know if this is still a problem these days with CCD guiding.
With a full 24x36mm frame there was also some vignetting of the frame from the OAG mirror. Would not be a problem with smaller area imaging chips.
Guide scope. Biggest hurdle is differential flexure causing star trails. Making everything as solid as a rock helps but when there is a reflector telescope in the equation, especially at long focal length, it can be hard to completely get rid of.
I'm not sure why you are trying to match f/ratio? I'm not sure what the correct formula is for guide scope focal length. It may depend on the tracking chip, or in my case my eye.