If you need a basic golf gps device then I would go with the Garmin G3. The G3, G5, and G6 both have pre loaded courses, no annual fees, and displays the shot distance which are the most important features. One of the advantage that the G5 has is that it has a bigger screen of 3 inches; G3 and G6 have a screen size of 2.6 inches. There are other features that you might like in the G5 and the G6 though.
You should read and compare the specs and price before figuring out which one is best for you. This article is pretty helpful comparing the different Garmin models:
http://deviceraters.com/garmin-approach-g3-g5-g6-comparison/