For really slow speed, an HDS with the Point-1 antenne and compass (heading sensor) is the best option. (At a price though)
The 2D-sonar in the Elite-7 and the new Elite-5 are the same as in the HDS-series, and that is very good. The HDI is basicly a slightly downsized version of the StructureScan, and that is also a very good product.
The main difference between the Elite 7 and the HDS-models are compability with other electronics through networking, the Elite has as lower processor in it's chartplotting-module and that the Elite is not compatible with Navionics Platinum cards (only the Gold). The Elite 7 can be used with Insight Genesis and other software for making your own depth charts.
In other words: the Elite is a very good unit if you don't want more screens, Sidescan, engine data, music, radar and other "toys" to work with the unit.