Hi Daniel, welcome to IIS

You'll be lucky to get any viewing (decent, that is) during summer. Too cloudy and humid!!!.
12" dob will be a great purchase and you'll get clear images (the mirrors don't really make a difference). It'll be a better image you see in most respects, to one you'll get out of refractor. For one, the 12" will gather much more light and give you a relatively brighter image. You'll be able to see more, and deeper, because of its much larger light gathering capacity.
As far as seeing things that no one else has, that's entirely up to luck and how often you observe. Plus, what you intend to observe. You never know your luck in a big sky

Doesn't matter what size scope you have, there'll always be surprises somewhere and it's just up to you and lady luck whether you see them or not. Although, the larger the scope, generally the more you will see (up to a point).