I wil reorganize my GPS application, so, I must choose now between UTM (regions UTM borders) or LAT_LON (ddd.ddddd more global) data for calculating all sort of things. At the moment I use UTM, but I was thinking to use LAT-LON data strait from the $GPRMC sentence...
Once started, I have to stick with it for the rest of the app. Is this the best choise for further development ???
I suggest to work with UTM, as long as the application has to deal with distances of less than 100 NM because the grid of UTM is rectangular so calculating distance or azimuth is straight-forward, while with spherical coordinates the algorithm for range or azimuth is complex.
If the application deals with larger distances - you must work with GEO
As derez wrote working with UTM coordinates is usually simpler.
One disadvantage of UTM is that it is very inaccurate to calculate distance and bearing between two coordinates on different zones (even when the range is small).