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Hi Andrew,
Yes, as a general rule, a 8" Newtonian has more light gathering ability than a refractor (unless the refractor has a diameter of more than 8"!).
The exposure time is dependent on the f ration, just as a f2.8 lens is "faster" than an f4 lens, a f4 or f5 Newtonian will require much less exposure time than an f6.5 or 7.5 refractor, even if that refractor is 8" in diameter (when 5" refractors are considered big).
As for goto, it's simply a set of computer controlled motors. Once you enter key data such as your location (latitude and longitude) date and time, the onboard computer will be able to power the motors to point the mount (and therefore the scope attached to it) to a preset locations in the sky (then you will need to fine tune, depending on how accurate your polar alignment is).
Just to confuse things, you can get goto dobsonians (simple dobsonians don't have the goto kit with computer and motors, but like everything else, you can upgrade non-goto dob to a got dob, clear as mud????).
The key here is a goto dob is on an AZ mount, where the mount needs to track in both altitude and azimuth planes whereas an eq mount only needs to track on the right ascension (RA) plane once the declination plane (dec) is set via the altitude bolt on the eq mount.
You can do planetary imaging and short exposures of 30-60 secs on a goto AZ mount, but for lingers exposures, a eq mount is almost a must.
Hope that clears things up a bit!
Bo
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