Re: humminbird 757 or 787
I have a 797 which is a 787 + SI. I believe you are right, the main differance is the screen resolution. Download both user manuals and compare all the features to confirm. To tell if the screen resolution makes a differemce for your eyes, visit a local store with both on display and look at a detailed section of the gps map.
Also, email tech support at Humminbird, there maybe parts from your current unit that will work with a 757 or 787. I was surprised to find out my old 3-beam transducers and quick mounts (with mod) will work with the 797. This might allow you to switch/swap around with different boats.