Preparation for tournament

So, tournament game coming against a player rated maybe 150 points lower than me. I always open with 1.e4, he always plays 1.e5. Against the Ruy (my opening) he plays 3... a6 4. Ba4 b4 5.Bb3 Nf6 … etc. He seems to have that line pretty well studied.

My options are:

1) Play a King's Gambit, having prepared something to take him out of comfort zone and enter a position I have forseen

2) Play 3.Bxc6, to enter a very long, positional game where maybe I have a slight edge because I'm higher rated.

3) Enter his mainline and prepare something against it

What would you do, and why?

Unless youre an elite player, or at least an IM/GM, openings do not matter. 

So, tournament game coming against a player rated maybe 150 points lower than me. I always open with 1.e4, he always plays 1.e5. Against the Ruy (my opening) he plays 3... a6 4. Ba4 b4 5.Bb3 Nf6 … etc. He seems to have that line pretty well studied.

My options are:

1) Play a King's Gambit, having prepared something to take him out of comfort zone and enter a position I have forseen

2) Play 3.Bxc6, to enter a very long, positional game where maybe I have a slight edge because I'm higher rated.

3) Enter his mainline and prepare something against it

What would you do, and why?

At 1600 level changing your opening and "preparing something" in order to get the opponent "out of comfort zone" is a bit like an amateur athlete buying super expensive shoes and aerodynamic clothing in order to break the 14 seconds mark in his next 100m race.

BS

Openings do matter. I'm playing at 1600-1800 level, and getting a better position after the first 15 moves is an important factor. It's also a psychological advantage to enter well known positions while your opponent struggles to figure out the plans.

If openings don't matter, why does every player have better statistics with white than with black, at any level? Just look at your or mine chess.com statistics. It's easy to see

At 1600 level changing your opening and "preparing something" in order to get the opponent "out of comfort zone" is a bit like an amateur athlete buying super expensive shoes and aerodynamic clothing in order to break the 14 seconds mark in his next 100m race.

Considering that you are the higher rated player, who plays always 1. e4, and has white in this game, I don't think you need to change that much. So if Ruy Lopez is your bread and butter, I would probably select option 3 and go for the main line with a deeper idea somewhere in the middle-game. Trying to prepare something completely new like King's Gambit can backfire quickly, and there are quite a lot of lines to study for. Of course option 2 is fine as well, but despite having an advantage, it's not the easiest one to nourish for white. You'd be in it for the long haul.

But since you are an e4 player, I assume you know your Ruy Lopez pretty well. If I were you, I wouldn't mind challenging him on a mainline Ruy. Not to mention that if it's his main choice of defense, you are much more likely to end up in a position you have studied beforehand (compared to going for something completely different).

Like everyone said play your strong point (which is Ruy Lopez here)  in this tournament(when both of you use the Ruy Lopez being the higher rated there is more probability that you will win). Since you are considering to play a gambit - learn some key ideas of it after this tournament (and play many games in chess.com and analyse them) so that you may use it for later tournaments as a surprise weapon. 

If he plays that exact move order in the Spanish with a very quick b7-b5 there are a couple of potential problems for him. One is that a very quick Ng5 might be annoying.

Another is that he’s not as well placed to meet a quick d2-d4 as he is in some variations. This is why Black usually delays ...b5 until it’s really needed, for example when White has defended the e4 pawn and is legitimately threatening to take on c6.