50 and 83 will also have a wider angle so will give a bigger picture. Handy if you are not sure what is there , ie. better to explore then, when you find something use the 200 to give greater detail as long as it isn't generally in very deep water in which case the lower freq. will show up better.
Also having a wider cone angle is useful when searching for bait schools or marking fish in the water column.
I run my sounder (furuno 585) on dual freq almost all the time. Best of both worlds and we are always exploring when moving.