When I was setting up my boat I went through the same dilemma, but came to the conclusion that mounting the fish finder on the middle bench made more sense. I do a lot of trolling, so I had to imagine it this way. Sitting in my seat on the rear bench, kicking back with my feet up on the middle bench, trolling along, do I really want to be looking down at my fish finder or looking ahead at both the fish finder and where I'm going. True, I have to lean forward to adjust it but once set I usually don't have to touch it anymore. So I mounted it on the starboard (right) side of the middle bench and it works good for me and it's out of the way so it doesn't get bumped, kicked, fish goo'ed, etc.
just my .02 worth.