I have never used the Boss loopers, so I don't know what I'm missing. With the BeatBuddy, however, getting a looper to sync with the drums is very difficult, with the Boss loopers you have to press your foot down at just the right time. You would mess the timing up 19 times out of 20 before getting it right!
The Ditto Jam syncs with the beat buddy, or whatever drum machine you use with it, it sort of stretches or flattens your loop to play in time with the beats! So you don't have to worry about the timing so much, get it near and the Ditto will do the rest.
If you are looking for the best drum machine around then go for the BeatBuddy and Ditto Jam but if the looper is more important to you then you probably don't need the BeatBuddy.
Currently I mainly use the BeatBuddy's metronome and use the Ditto to listen back to my playing...but that is because I joined a covers band in July and my practise sessions are mainly taken up with learning songs.