Remember that 6x12 and 6x17 are wide Panoramic formats, a 90mm lens is a very practical option and makes a good standard lens for 6x17.

My personal preference is a 75mm Super Angulon, neither the 90mm or the 75mm give exaggerated perspective on 120 film, either at 6x12 or 6x17. After all 75mm - 90mm lenses are standards on 645/6x6 and 6x7 cameras.

Jamie makes a good point, one of the reasons for shooting 6x17 as opposed to 5x4 is to get everything you want into the image without an excess of foreground and sky.

Ian