Just my 2 cents worth. I have the HB 300 series color combo GPS FF on the console (3.5" screen) and an older B/W 535 on the front. It has a 5 inch screen. I find myself relying on the 5" screen when I am trying to find BPs or fish under bait, etc.
The 300 I bought due to budget constraints, but if I had it to do over I would never go less than a 5" screen. If you only have a 3.5" screen and split the image between GPS and FF, it is so small you have trouble identifying what you are looking at. I would rather have the larger screen than the color, but the best option is the color in at least a 5 inch screen. I am saving for a SI unit now, gonna be a while.