I guess this has already been said but I like to put in my two cents worth anyway.
   To me, old time music is a descendant of primarily Celtic and English music of yore.It is much more basic, less intricate than the later bluegrass.The latter would appear to be inventive and varied both in lyrics and instrumentation.
   Having said all this, I love both styles.For old time some of my favourites are:
                The New Lost City Ramblers
                The Carolina Chocolate Drops
                Olabelle Reed

    Bluegrass: The Del Mccoury Band
                Flatt and Scruggs
                Ralph Stanley and the Clinch Mountain Boys