I actually own a 6x12 and can speak from experience as opposed to speculating about them. You can easily hand-hold a 6x12. People USED to hand hold 4x5's - remember all those photo journalists with Speed Graphics? I use my 6x12 hand-held about 80% of the time.
My 6x12 back has a dark slide so the back can be taken off the camera with film in it. I also have a ground glass for the camera. Nearly useless with the 45mm lens I use, as you need a center spot filter and the light fall off at the edges of the image makes the ground glass nearly useless.
This is not as much of a problem with a longer focal length lens, but you have to decide what you want - a near panoramic format or a really wide field of view. I wanted both, so I got the 45mm and it provides a 110-degree field of view. You can look at the FOV for 4x5 lenses and figure the FOV on a 6x12 will be very close.
If you don't get a camera with an optical viewfinder, you will have to use the ground glass for composition - just remember the caveat about really wide angle lenses and light fall-off.
Otherwise, shoot 4x5 and crop the 6x12 out of the image.