They are basically the same unit except you can add other peripheries, like GPS to the Matrix. They are both dual frequency and you may not be able to select one or the other - narrow or wide - on either of them. For the way I fish I much prefer a single, narrow cone angle. I think you get a clearer picture and when you see something come on the screen you know right were it is - directly under your transducer - unless you are in very deep water. I use a Matrix 12, which is basically the same as the 525. It is a single, 20-degree (narrow) trandsducer and I can easily see a 1/16th-ounce jig and identify fish that are down in our bamboo condos.
I believe you can connect your Etrex Legend to the Matrix units but I'm not sure what the advantage would be. I use a Map76 handheld in a Marine Mount next to my graph so I can see them both right in front of me while I'm hovering over our condos.
It depends on how you fish but if you use both sonar and GPS at the same time I think it's better to have two separate screens or a large enough screen that you can display the GPS on one side and still have enough screen left to properly display sonar readings.