I owned a Stratus 2 for a couple years, a GDL 39 for a year and now have a GDL 50 which is the no XM version. In my experience, there is a world of difference between the 39 and the 50. The GDL 50 is very stable, very consistent, very accurate as was the Stratus 2. The GDL 39 was not any of those, a rather poor attempt to match the Stratus 2. With the GDL 50 Garmin caught up with the STratus 2. It is a very good unit. Garmin says the technology in the GDL 50 is way superior to the GDL 39. They are correct.