Although my experience is limited, I feel like an expert on this subject. Go with size, and bigger at the bow. Try not to let $$ be the final determining factor. I started with a 3 or 4" on my new boat 2 years ago. I upgraded to a Lowrance Elite5TI shortly after getting the boat, mainly because I was watching the $$. Although the unit worked perfectly fine, I could not see the screen details unless I was right in front of it at the console. It didn't do me a lot of good when fishing from the bow. I have upgraded again, bit the bullet on the cost, and put 2 Lowrance Elite9 Ti's, one at the bow and one at the console, and have the linked both to my Motorguide Xi5. The upgrades weren't cheap, but I wish I hadn't done the in between step. If you can wait and save some money, I'd hold off so you don't make the same mistake that I made.