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The biggest factor in all imaging is the Quality and performance of the Mount. Spend more on the mount and start with small wide field Refractors and move up to Long Focal Imaging with a 8" SCT. Thats the standrd advise. But very rarely can people afford or do they even if they can spend more on mounting than optics.
Not sure that is a fair comparission, the Dob is not really a Imaging platform. That is without any modification Imaging is very challenging. Planet imaging can be done, but requires a great deal of paitience to hand track.
Eqatorial Tables/Platforms can be uses to give tracking, which would defineltly improve Planet imaging performance, but DSO imaging would still be a challenge. Alt/AZ motorisation for tracking and or Goto would further improve Planet imagin performance, and you could do limited short exposure DSO work before field roation gets you.
The 8" SCT in Alt/Az mode is a very portable setup for Visual and Planetary imaging. Limited DSO imaging as field rotation will hinder you in Alt/Az mode. But then you could ad a EQ Wedge and do DSO that way. Generally speaking the Alt/Az mounts have very poor Periodic Error performance and will require Guiding for DSO imaging. Further even with guiding there is a upper limti to what you can do with these. But many users with lot of love and care and fine tuning have produced stunning results. The best upgrade if your serious for DSO on this type of setup would be a camer with adaptive optics setup. I have seen some awesome shots from such setups.
8" on a EQ mount, is perhaps the best of the lot. As with the Alt/Az to really to long exposure DSO work you need a Wedge, that turns that into a EQ mount. So this is perhaps the best you coudl go with. But when you start dealing with Wedges and EQ mounts you loose some of the portability factor fo this type of setup over a Dob.
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