Some ant here and there might eat a dead aphid or two, but they seem to actually have a symbiotic relationship, more or less, with the ants using the aphids for sugar, which they actually make, believe it or not, and then in turn protecting the aphids.
So, no, neither is helping you. Washing the plant off with a very diluted soap and water solution will get rid of the aphids.
There are sustainable ways to keep ants off the plant. Getting rid of the ones on the plant and the balcony and placing a dish of water on which your plant sits would be one way to go. A deep dish that is bigger than the pot so that the ants can't swim to the island (ants are pretty clever and resourceful) would help.
Also, I just saw this on this same forum:
Ants and aphids - which should I fight first