What the advantage over having BF over SA when you can do SA burst ?
SA Burst is shooting bullets for a full action phase so your recoil will carry over to the next action phase.
Burst fire is only shooting bullets for a simple action so recoil will reset to the next action phase.
(You also get to use the other simple action for stuff like take aim, take cover, reload weapon or whatever)
BF weapons also have the option to fire two individual 3-bullet bursts in the same action phase as one single attack, which give the target a defense roll modifier of -5.
Long Burst and 6 bullet Full Auto are not really the same thing though. Long Burst fire two individual 3-bullet bursts (press trigger twice) while 6 bullet Full Auto spray 6 bullets (press and hold trigger for a simple action). Long Burst can also be used to fire the two individual 3-bullet bursts at two individual targets (but both targets need to be at close or medium range) while the 6 bullet Full Auto only can be used to fire at one target (at least until we get additional rules that let us spray n pray Full auto in a cone attack)
(.... then again, we have rules for spraying full auto in a cone attack. In SR5 that is called "Suppressive Fire").