[raymo]

You've got several different aspects of scopes to consider here.
1/ A 12" GO TO Dob is one big mother to be moving around frequently, but
fine if it is going to stay in one place, or you are built like Arnold
Schwarzenegger, and have a fairly roomy vehicle. There is a fine line
drawn where a scope becomes large enough to make you sick of
moving it around, and ends up being under used, or even abandoned
all together.

2/ For serious astro imaging the 10" SCT is more suitable, as the
tracking is better, and the short tube is more wind resistant, and
there are counter weight systems available to counter the weight
of the imaging equipment you will hang off the scope. Quality
imaging also demands really good collimation, and the SCT will
retain it's collimation for far longer than the Dob. Neither scope
is the best imaging scope around, but I would go for the SCT every
time.

3/ As far as what you can see through each scope is concerned;
You won't see much colour in nebulae with either scope.
With average colour perception you can see a faint but definite
tinge of green in M42 for example with the 10" SCT, and it is only
marginally stronger in a 12" scope. I find the only significant
difference between the two scopes is when observing dense star
fields, and very faint deep sky objects, when there is a noticeable
[though small] improvement.
I hope this helps
raymo