Btween Shots is correct, the technology is advancing so fast it's hard to keep up with all the new features.

First, decide your price range. Units run from under $200 to over $1,000. Next decide the main reasons you will be using it. If all you want is a simple depth and fish locator, the low end units will be fine. If you want a little more, such as a wider signal "cone" (the amount of area covered) or multiple signals, split screen, etc then go for a little more advanced system. If you want some of the new features, such as the ability to connect an underwater camera to it and advanced GPS, then shop for the high feature systems.

Don't forget the most important thing: knowing how to read it. You can have a $2,000 feature rich unit but if you don't know how to use it or how to read what you are seeing it won't matter.

It won't be long before these units will have fish ID ability that is fair to good at getting species correct. Sonar units that do this already exist but are quite expensive and mostly used for research (I've seen one of them and it's quite fascinating). As the technology improves and comes down in price these features will be added to the recreational market systems.