CSV carries no formatting, formulas or multiple sheets, and Excel often misreads its dates and encodings. GPX is read by practically every GPS device and fitness app. CSV is at home in moving tabular data between systems; GPX is the usual choice for GPS tracks, routes and waypoints.
Every format is read into plain GeoJSON features and written out from there, so coordinates, names and properties carry over wherever the target has a place for them. The first row must name a latitude and a longitude column (lat, latitude, lon, lng, longitude and similar); name, elevation and time columns are recognised, and every other column becomes a property. A row with no valid coordinates stops the conversion and is named. Points become waypoints and lines become tracks; GPX has no areas, so polygon outlines are written as track segments. Elevation and time are kept. Coordinates outside the latitude and longitude range are refused, since the file is probably in a projected coordinate system.