Have you looked at the size of objects you wish to image & what image circle you will require in relation to your camera sensor size?
Longer focal length will result in smaller image circle; for example, when I started my imaging journey with an 8" f10 SCT, I was unable to capture objects such as m42 & eta carina in there entirety. Even with a 0.63 reducer, I could only just squeeze them in. I now image with an f6 80mm triplet & 130mm f5 newtonian & they happily cover my interests.
Longer focal lengths also usually require longer capture times.
If you want to get a feel for fov, put your sensor specs & each scopes specs into stellarium, you can even add in the reducer. Then pic targets & get a feel for how they will frame.
Also, there are ccd calculators that will show if you are under or oversampling with particular combinations, I think there is one on the bintel site.
Double vs triplet, well I think that depends on the quality of the glass used. If choosing between two scopes of equal quality, I'd choose a triplet over the double assuming I could afford it...
I'm sure other, vastly more experienced imagers will chime in with more advice but, that's a snapshot of what I've learned over my journey that relates to your questions.
Hope it's helpful
Edit: I see they already have...
