1st, Try this:
Take some 8x10 mat board(doesn't need to be archival
), and cut a 'window' out of each.
One for the 6x17(Linhof is the only 'true' 6x17, Fuji is more of a 6x16 or so) format, one for 6x12
Take them with you when you go out, even if you don't plan to photograph. Frame up things using the cards, and see which format you prefer.
REMEMBER: With 6x17 you can always "chop the ends off" to get 6x".." format you want. 6x12 limits your 'choices' as it were, unless you purposefully WANT the 6x12 perspective straight out, no cropping.
Linhof makes a beautiful product, the 617SIII is probably one of the most beautiful products I've ever held in my hands, period. h GX617 system looks and feels like a toy compared to the Linhof, IMO.
But definitely try the viewing cards approach, much cheaper and easier than buying "another" camera system.
Watch this video to see what I'm talking about:http://www.youtube.com/watch?v=tSEo9dBlvE4