i dont know why you are so caught up on "flutter", as it makes no difference whatsoever.
wastegate's work by diverting un-needed exhaust around the turbine and out an outlet (external wastegate in this case). Now the computer needs to tell the wastegate when this valve should open.
There are 3 different ways for the car to communicate with the wastegate
1. wastegate spring pressure - once the exhasut gas reaches or exceeds this spring pressure, the wastegate opens, and all extra air leaves via wastegate. Can only open at one pressure (the pressure of the wg spring).
2. MBC - acts just like a wastegate spring, except you can control the psi at which the wastegate valve opens/closes. can only open at one pressure (PSI) that the user sets it at
3. ebcs - uses computers to control the pressure at which the wastegate opens. The nice thing about ebcs is that they can change the level of psi that causes the wastegate to open, unlike MBC or wastegate spring. This allows you to run higher boost at low rpm, and taper off boost at higher rpm. this is verry common on subarus running smaller turbos.
It shoul be noted this can not be done on a MBC. with an MBC you only get one target and that is the only level of boost you can run in any gear at any rpm. With EBCS you can run different boost at ifferent rpm. This allows for greater control in tuning, and often more power.
Now that you understand the ifferences, you need to know how they work. MBC operate by opening a valve at a set pressure. the air flowing through creates a whoosh soun
an EBCS operates like a solenoid, by opening and closing a valve really fast to acheive the target pressure. This rapid opening and closing is what causes the fluttering sound.
THE Fluttering sound IS NOT BAD. You simply need to know how all these parts work, and what their function is so you can understand what is going on. hopefully that cleared something up, and you learned something today.
personally i run an EBCS, because it allows better/more boost control. that fluttering sound is suppose to happen and its nothing bad, so dont freak out
if all i cared about was "whoosh" vs "flutter" then i could get an MBC, but i care more about performance and the overall power and tuning capabilities of my car so i choose EBCS.