+10 plasma would be better because its specific to the arrays, but then you would end up getting unbuffed if you found a better array of a different type... like Tetryon.
actually its 1 less then the +11 beam one.
just because its for a specific damage type doesn't mean its better. Yes its usually better as comparing a +beam and a +damage type of the same quality and rank the +damage type will have a higher modification but in this case the +beam one gives a 1% bigger increase then the damage type.
I would go with the +11 beam unless you were using plasma torpedos as well then the +10 plasma MIGHT be better in the end since it would effect all weapons, in the end though the difference is so minute it really doesn't matter much