Thats a close one. I'd have to say either manpower or tactics. manpower because the generals now paid their soldiers and i believe provided armor, weaponry and equipment. This also resulted in the armies being more loyal to their commander. On the other hand, good tactics were developed to out manuever the enemy. (someone please correct me if i'm wrong)
Off topic (sorry): speaking of tactics, was the phalanx considered obsolete by the time of marian reforms or no