Your thinking is on the right track, Kirk.
I have an 8" SCT and many year ago I used to own an 8" Newtonian. The 8" SCT does have a longer focal length, but you can add a focal reducer to it to increase FOV and make it faster.
Basically a Newtonian will give you better contrast than an SCT because it has a smaller secondary obstruction. So you will see a lot of both planetary and DSO specialists using Newts - the planetary guys will use barlows and TV PowerMates to extend their focal length.
The SCT is more portable than a newt and can be a bit more versatile depending on what you went to do. E.g. with a couple of focal reducers and a Powermate you can have one scope that changes from f/3.3, f/6.3, f/10 to f/25 say. If you got one with fastar or hyperstar you would get something like f/1.8! The SCT also has more focal range so I can achieve higher dispersion rates out of say a Staranalyser 100 spectrometer
(which I curently enjoy
) and it's possible to use an off axis guider without having to modify the scope. But the trade off is you will never get the contrast you get out of a Newt... that may not mean a lot until you get to the expert end of DSO and planetary imaging and yet a lot of expert imagers use SCTs (e.g. Damian Peach).
The SCT will do the stuff you say you want to do
. The planetary stuff will be relatively easy to achieve - the DSO's might be more difficult or disappointing depending on the mount you have. To do DSO's justice you will need to guide, which probably means a heavier mount (say EQ6 or G11) and guide scope unless you tackle using an off axis guider.
Al.