I have a Garmin echomap 53dv with chirp. The main differences between mine and the 73sv is screen size (mine has a 5" screen) and the 73sv has side scan (mine only has downscan). Other than that, both are very similar with having all the other features and software.
I absolutely love my Garmin unit and highly recommend going with the 73sv. I specifically chose Garmin because I use my boat exclusively on Lake Powell and other sonar manufacturers could not compare with the mapping and navigation abilities that Garmin has for dealing with fluctuating lake levels. Here is what I like about mine.
1) The ability to shade the map chart for different depth ranges in various colors is a huge advantage. I posted some tips on how to shade the map chart at this link:
If you have a Garmin sonar unit that has topo maps for inland lakes installed, it may allow you to shade the map chart for different depth ranges. Here are some tips on shading the map chart so it will correspond to the actual shoreline and highlight shallow areas to avoid while underway. Map...
wayneswords.net
2) It uses very little battery power. I leave mine on all day and never had an issue with my cranking battery.
3) It is very effective in finding resting schools of stripers on the bottom at 80-100 feet deep. I use the split screen feature. The traditional sonar shows the surface to the bottom while the downscan is zoomed into the bottom to just show the area 10-20 feet above the bottom and adjusts automatically for depth. It displays individual fish very well and I can also see my 1 ounce spoon bouncing on the bottom at 100 feet deep too. The sonar is that clear.
4) I have not used the Quickdraw Contours feature to create my own maps yet. The mapping software that comes with the unit shows the bottom of Lake Powell with contours at 10 foot intervals. I have found that adequate for my needs.
5) I have not tried connecting the Garmin to my trolling motor and other devices. Wish I could. The Garmin is capable, but my devices are not.
6) I decided not to go with side scan due to the extra cost and because I felt there was not much need for side scan on Powell.
7) I had to get the 5" screen to replace a Lowrance unit that was previously installed in the dash of the drivers console. I would prefer a larger screen, but had limitations for mounting. The Lowrance unit was relocated to the bow and the Garmin was installed at the helm. The 2 transducers do not interfere which each other if both sonars are on at the same time. The transducers are about 18 feet apart.
Hope this helps.