Rental income per month is not the only consideration you should be looking at
How you feel the value will hold up, will it appreciate, etc
I am sure you have considered these, but on the specific point you raised re income per month, you should also conisder vacant periods
City centre apartments are extremely rentable and will rent in a matter of hours, when advertised properly, where as suburbs are almost always a little slower
Try placing an ad in the Herald and see what response you get from both, it'll cost you roughly €35 each (roughly one day vacancy)
City centre apartments have very little vacacny periods