Hi Matt, the F-ratio is certainly important if you want to do planetary imaging through them. A 5x power mate would be good for the F5 newtonian in that case (F25 is a good f-ratio for planetary imaging), but for the 9.25 it would be running F50 and only in the most spectacular seeing and/or using a camera with large pixel size would this be viable (wouldn't let you use many eyepieces with 2350mm fl either). The 2.5x might be a good choice (I recently bought one of these) as it would give you F25 on the C9.25 (theoretically more with an extension tube) and would still give you handy additional amplification for use on the newtonian.
cheers,