I've used 6x17 (Linhof, Fotoman, Longfellow) and 6x12 (Horseman, Linhof) but not 6x24. In my experience, composition becomes easier up to about 6x15 (6x12 is a bit squat) then harder again. I'd also suggest that wide-angles are less useful than you might expect, but it's hard to explain why unless you try one.
Does the 6x24 just sound like a good idea, or will it enable you to do something you've been trying to do for a long time and can't do any other way?
Also, how big do you want your final prints to be? Crop 6x9cm in half to 28x84mm, and blow it up 5x, and you get a picture 420mm (near enough 18 inches) wide.
Finally, how big a chunk of film can your enlarger/scanner handle?