They both use a justu to increase their speed, A uses the lightning shroud, while Minato uses FTG, it doesn't matter what kind of jutsu it is, a jutsu is a jutsu regardless of the nature.
Using FTG Minato is faster, the manga proved that, and A admitted himself.
Minato without FTG and A using lightning release, in this case i must say that A is faster in terms of foot speed, but Minato still has the better reflexes and reaction speed, i mean it's only logical that he does, since he is so proficient at using FTG his reaction time is far beyond a regular shynobi, just imagine having to adapt yourself to different surroundings a hundred times in mere seconds.
And finally when neither of them uses any jutsu then Minato is still faster. At least this is how i see it, there is no proof for this and no one can say for sure who takes this one.
Please use the search button next time, this has been discussed sooo many times before.