GPX vs. CSV question

Start by reading the FAQs. As you have a 3790 a lot will depend on if you have paired your phone to the unit and would like it to dial phone numbers embedded in some POI. This can only be done with a GPX file.

As to content, either works but the GPX format gives the maintainer much greater control over how the data is displayed on the screen. There is another subtle difference when looking through your files. A GPX file will display not only the name field, it will also show the street address just below the name. CSV files only display the name.

Load one or the other, not both - and don't worry about the difference in file size. What you download is not what goes into the unit. Garmin's POILoader reads either file and then compiles what it finds into another file that is stored in the unit. Tests have shown the stored file, a GPI file is always about the same size whether GPX files or CSV files were used as input.

Being able to dial phone numbers to me is probably the biggest advantage over the .csv files.

You must use POI Loader to put either the gpx files or csv files on your GPS as an Extra>Custom POI item. Note you must have all gpx/csv files, along with a matching bmp file to get a nice symbol on your GPS map display in the same folder structure. I use:
POI/H-D dealers.gpx
POI/H-D dealers.bmp
POI/roadfood/bikereats.gpx
POI/roadfood/bikereats.bmp
POI/roadfood/feastingonasphalt.gpx
POI/roadfood/feastingonasphalt.bmp
POI/xxcameras/speedcamera.gpx
POI/xxcameras/RLCamera.gpx

That way the menu looks like:

H-D dealers
roadfood
xxcameras

Note these last two are category lists that open up another level POI list.