I use a Celestron 8" Edge HD w/ a 0.8 reducer for planetary nebula's (pn's) and at 1600mm fl it is probably a little too short for a lot of pn's. However, one also has to consider sky conditions.
An ATIK 428Ex with the above scope and reducer yields 0.58 arcsec/pixel (as/p) which is oversampled for my normal 1-2 arcsec/pixel sky. An ATIK 314L @ 0.83 as/p would be a pretty good match.
The above combo would yield a full frame image of M27 and be too short for M57. You can't win them all!
A 10" Newt @ f5 is a nice fast scope and good for a large number of targets (I have a SkyWatcher 10" f4.
. I also find it hits my sweet spot for tracking, guiding and polar alignment; any higher focal length and the whole configuration starts getting a bit shaky.
Aim for your sky conditions in terms of arcsec/pixel and you normally can't go wrong...
bwa