Fromper wrote: "the choice of opening isn't going to determine the outcome of any game below master level"
That's absolutely false; the moves 1.e4 e5 and 1.e4 c5 will lead to entirely different middlegames and endgames at any level of play, and if as an amateur you know the typical strategies and tactics in a certain structure better than others, your choice of opening is extremely relevant.
3.Bb5 pressures the e5-pawn indirectly and thus fights over the center. 3.Bc4 doesn't do this and is exposed to ...d5 or ...Na5 ideas. If the Bb5 gets pushed back to b3 by a6/b5, white will have provoked weaknesses and put his bishop on a better square than c4. Therefore, the Ruy Lopez is objectively much better than the Italian. It's not that hard.