[Elecraft] Why need a balanced line tuner?

[Karsten Eppert(DK4AS)]

I am happily using a doublett-antenna that I feed with a 600 Ohm 
ladder-feedline.
When I came to decide, what kind of tuner to use, I could not find 
anybody to give me an answer to my questions that would satisfy me:
-   use a balanced tuner, like the MFJ 974 H (which I finally purchased)
-   or use a standard-unbalanced tuner with a toroid-balun between the 
tuner and the feedline.

In fact, the balanced tuner has a balun at it´s front end, looking 
towards the transmitter. Other tuners have the balun at their backdoor, 
looking towards the feedline.

What is now the advantage/disadvantage of which concept? Can anybody 
explain that to me?

Yes, I am happy with my MFJ, but being an Elecraft-maniac in the 
meantime, I would like to have the autotuner, which would need the extra 
balun.

Thanks to everybody undertaking the effort to answer me.

Karsten
DK4AS