It has nothing to do with barrel length.If it was up to me i would go with the 18 inch barrel for all riflemen.TAR has 18 inch barrel, CTAR-15 inches,MTAR-13 inches. You can have any barrel length in a MTAR from 10 to 18 inches.
WITH THE SAME BARREL LENGTH :
The MTAR has a shorter overall length than TAR/CTAR
It's lighter.
It also has a shorter length of pull.
The butt stock is less bulky ( height ) .
The position of the mag release and the charging handle allow quicker reloads and stoppage clearance.
The shape and size of the bolt release reduce the chances of premature/accidental bolt release.
The handguard+charging handle location on the MTAR allows rail attachment on all sides unlike on the TAR/CTAR.
The handguard +charging handle location on the MTAR allows the barrel to free float.
The handguard on the MTAR allows the use of telescopic sound suppressors
or shorter barrels than 13 inches with regular sound suppressors.
The safety switch on the MTAR is easier to operate.(45 degrees)
You can remove the pistol grip therefore the size of the trigger guard is up to the operator's choice.
Trigger pull is much better.
These are only a few reasons why I would choose the Micro Tavor over the regular Tavor as a bases for an entire weapon family(rifle,carabine,DMR,GL)
If any of the listed advantages are unclear or not obvious I'll explain in further details.