I have always had seperate fishfinder and gps, I really never liked the thought of switching back and forth on the same screen. My brother had a fishfinder/gps and I didn't like it too much (I gave him my Garmin unit and bought a new furuno unit) I like to know where I am going and I also like to watch for fish and different structure as I am going. The best day walleye fishing I ever had on my own boat could have never been if I was switching screens. I was running from north west of rattlesnake and just came around about 1 mile south of South Bass and right in the middle of the Miller Ferry Line I marked a bunch of fish. I shut the boat down and we started casting, 1 hour 15 minutes and 38 walleyes later we were heading back to East Harbor. This was a few years back when the limit was still 10. I always tell this story when somebody asks me whether to buy a multi unit or buy 2 seperate units.
I have a fishing buddy in the marina that has a standard horizon gps with a 10 inch screen, it is a very nice unit and very easy to see and operate. He also has a furuno 620 fishfinder too.
The transducer I have for my furno 620 is the SS60-SLD. The boat wasn't mine 24 hous yet and I drilled a 2 3/8" hole in the bottom of it, but it work great at all speeds. This same transducer can be used with the 585 also.