Ruy Lopez opening

just take the knight. It wastes a tempo for black, and then castle on the next move. if bishop goes to d6 to defend the pawn, then just move d4

What does that mean? lol Just trying to lean the game better sir.

It is true that, to a large extent, the best choice depends on the person making the move, and nobody can make the decision for you. However, it is possible to provide you with some of the pros and cons. The Robin Shaw quote gives some indication of what one is going for by playing 4 Bxc6.

"The Exchange Variation is an 'endgame opening'. … The dream is a winning pawn ending - a later d4 will swap d- for e-pawn when White's healthy kingside majority will be able to create a passed pawn but Black's queenside will not. This is the traditional view of the Exchange Variation and there is a lot of truth in it but white players should not neglect the possibility of a successful middlegame attack. Black players frequently take risks with their king's safety to avoid the dreaded ending." - IM John Shaw (2003)

https://web.archive.org/web/20140627024240/http://www.chesscafe.com/text/hansen53.pdf

While pursuing that sort of game, one has to keep in mind the potential danger of Black being able to effectively use the two bishops.

At the highest levels, 4 Ba4 is the more respected move, but, of course, you are not at the highest level and 4 Bxc6 has, from time to time, been suggested as a reasonable choice for an amateur who doesn't want to take on the intense study associated with the play of a line like 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Ba4 Nf6 5 0-0 Be7 6 Re1 b5 7 Bb3 d6 8 c3 0-0 9 h3. 4 Bxc6 is not the only reasonable way to avoid that line. A recent Ruy Lopez book, Ruy Lopez Move by Move,

https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf

http://marshtowers.blogspot.com/2011/11/chess-reviews-190.html

discusses the possibility that those with limited experience go for 4 Ba4 Nf6 5 d3 in order to avoid some of the more demanding possibilities that might arise from 5 0-0. We can tell you things like this, but a lot depends on how much work you want to do, what sort of position you like to play, and what your ambitions are. You may want to look at some games illustrating the various options.