As others have already said, the 12" will be far better for viewing galaxies, planets and nebula... The 6" mak is not a scope to be sneezed at, It will give great views, but the resolving power and light gathering is much much less than a 12". The 12" in its mount pointing towards the ceiling (with a slight tilt to minimize dust falling on the primary) takes up less than 1mē... perhaps a 10" or 8" dob (both of which have more light gathering power than the 6" you've been looking at..
Oh the options...
its a pain when there is an 'astronomical' amount of options to choose from isn't it.