I assume MAG is what we here in the US refer to as GMAW. If I'm wrong, please correct me.
GMAW using spray mode transfer produces a very fluid weld puddle and is limited to flat grooves and flat and horizontal fillet welds in most metals (aluminum being an exception). If you can "roll" the drum as you weld such that the groove is in the flat position, by all means use GMAW spray mode transfer. The process produces a deep penetrating, fluid puddle that wets in nicely. Provided you select the filler metal carefully you should be in fine shape with regards to mechanical properties.
However, if you need to have the capability to weld in all positions you will have to use GMAW pulse mode or FCAW with the electrode to provide the versatility you need. GMAW using short circuiting transfer is asking for problems with fusion type discontinuities because it produces very low heat inputs.
I would give strong consideration to FCAW because it does utilize a flux system that improves deoxidation, a slag blanket that reduces the cooling rate, and with careful filler metal selection it can produce good mechanical properties.
However, if the manufacturer has provided specific direction, you need to give their recommendations serious consideration.
Best regards - Al