One of the great things with HDS, that Elite does not support, is ethernet. With ethernet you can connect two (or more) HDS-units and let them share maps, sonar and waypoints. This means that if you start out with a 5, and end up feeling that you should have gone bigger, the second screen you buy will share with the first one. I used to run 2 HDS 5 and one 8, but now have one 5, one 8 and one 9 on my dash. Probably more screen-space then most find necessary, but very nice.
The 4:3 screens like 5,8 and 10 are very good for regular sonar, maps and downscan. The 15:9/16:9 are nice for splittscreen and sidescan.
I started out with HDS 5, and then it sort of took off from there. :D