The 12" will give you amazing views compared to your 5". As long as you aren't interested in astrophotography a Dob is the way to go for shear economical light gathering. I get wonderful views of Saturn and Jupiter with both my newt scopes. When you observe objects like Omega Centauri you will notice a huge difference.
When I bought my 10" SW Newt I thought I would sell my 8" but I love them both. The 10" is a mother to carry and setup on EQ6 mount at 14.6kg so quite often I just grab my 8" ( 10kg ) to go have a look see. Soon, however, I will have a permanent setup so we'll see then.
Have a lot of fun !
Steve
p.s. You may find you will need a step to view some objects. I have a small plastic step that gets me about 20-25cm off the ground, it's indispensable with my 10".