It depends on which camera that I will use most. If it's the DMK, I will go for a f/3 reducer for it maximizes the photographical speed. However, if I go for a DSLR, I will probably go for a f/6.3 reducer for a f/3 reducer cannot be used in that case.
Antares has a f/6.3 reducer which is relatively cheaper, and I've searched around and found that it actually has 4 elements like those from Meade and Celestron, so quality and design should be quite similar.
So, here is the situation:
1) DSLR： I will probably go for one very soon, should be before the total solar eclipse in August 2008, and I will be using it mostly with camera lens like Canon EF 200 f/2.8L and Canon EF-S 10-22mm, or maybe also a 50mm f/1.8. The only scope which I might use would be a Borg 45ED II, since I don't have a good enough mount to use it with my C5 or Sky90, therefore, it seems like the focal reducer plan does not have to include DSLR as a major concern, unless I go for a better mount.
2) DMK: I will be using C mount lenses for most cases, my existing collection includes a 8mm, a 25mm, a 75mm and a 150mm. The only telescope which I will use will be again my Borg 45ED II, and maybe probably the C5 given if I use a very strong reducer like f/3.
Therefore, from the above, it seems like a reducer is not very relevant for both cases.
1) Borg 45ED II: when used with f/6.3, it will be like f/4.55 effectively; when used with a f/3.3, it will be like f/2.4... maybe f/6.3 with my existing binoviewer corrector plus extension tube, it will be like f/3.7, not bad. So, it seems like the f/6.3 can serve both the DMK and DSLR for good results. At f/2.4, my Borg will be like a 108mm lens which is even shorter than my 150mm f/3.5 C mount lens, at f/4.55, it will be like a 200mm lens which could be duplicate with my planned 200mm f/2.8...
2) C5: I won't be using it with DSLR even with a f/6.3 for it's too long focal length for my existing mounts, with a f/3.3, I can probably use it with my DMK.
Considering the above factors, a f/6.3 would be more suitable for me, but if I go for a 200mm f/2.8, reducer is not needed at all.