I'm purchasing my first fish finder. I just bought a Piranhamax 175 from Bass pro until I seen Cabelas had a Humminbird 571 HD DI on sale for $129. Then I got to looking at other units in that same price range $100-150. I'm putting it on an older 14' fiberglass boat and will be attaching it to my trolling motor. I'm leaning towards the Humminbird 571 because of the sale price. Does anyone have any experience with this unit? Also, they have a Lowrance Elite-3x DSI is at cabelas for $129.

Make sure the unit has standard 2d sonar AND Down imaging. Those two views should be used together to decipher what you are seeing.

The humminbird has single and dual beam sonar. 

If you are looking for a lowrance with both broadband and downscan you are going to need a unit with the HDI transducer.  Also if you are looking for a gps sonar at a low price you might want to look at a vexilar t box unit and pair it with you phone or tablet with the navionics boating app and you can get a gps sonar for about 140.