short simple answer
Diatonic harp, Key of D if like playing straight forward folk / pop songs ( 1st pos )
Diatonic harp, key of G if like playing blues ( 1st pos of Key of G = 2nd pos key of D )
Solo tuned in key of D if you like playing melodies in key of D ( no accidentals unless bent notes )
Chromatic harp, key of D if you like playing melodies / classical stuff in key of D and need all the accidentals
Longer answer
You need to google more : )