Hi Brad.
Both the F5 and the F4 scopes will suffer from coma around the edge of a largish APS size DSLR or CCD camera sensor.
My F5 skywatcher certainly does, hence why I use a Baader MPCC on it with the DSLR. When I use the Atik CCD, it's sensor isnt as big, so it doesnt get as much coma (if any).
I would recommend getting an MPCC, if you do, get the "kit" one that comes with some extension tubes, allowing you to adapt it to different cameras and get the spacing right. I had to buy a 42mm spacer set to get it to work with the ATIK whereas the DSLR needed none, the MPCC just screwed onto the T ring.
F4 will allow "slightly" more exposure and brighter images than the F5 for the same length of exposure, however, the coma may be worse. Stick with the F5 in my opinion, it's a great visual/imaging compromise.
Also, I prefer the Skywatcher over the "Bintel" GSO rebranded scopes, but by all accounts the GSO reflectors are as good as the Skywatcher in terms of quality and longevity. Some may even chime in and say they are better!
6 of one, half a dozen of the other, but IMO you can't beat the sexy flecked black paintjob of the Black Diamond Skywatchers. Ooooh baby!
Cheers
Chris