As far as I know, there isn't a very clear demarkation between the two. The 1929 edition of the 20 volume edition of Oxford English Dictionary, for example, defines 'stutter' as 'stammer', and cross reference stammers back to stutter. So outside of medical parlance, the two can be considered equivalent. Generally, the medics seem to regard stuttering as hesitancies and stammering as repetitions, but I'd hate to claim that was universal.