Ruy Lopez Schliemann Defense

Hi I have a question about the Schliemann Defense in the Ruy Lopez.  At GM level, white taking the pawn on move four is rare.  My questions is why?  How does black get an advantage if the pawn is taken?  What are blacks best moves if white captures it? 

Taking on f5 on move 4 is one of the most uninspired options for white:

The most representative line goes as follows: 4.exf5 e4! 5.Qe2 Qe7 6.Bxc6 dxc6 7.Nd4 Qe5 8.Ne6 (8.Nf3 Qe7 9.Nd4 Qe5 repetition) 8. ...Bxe6 9.dxe6 Bd6 followed by ...Sf6, ...0-0-0 with easy play for black, white should be concerned about equality.

Black can also try the more ambitious 7. ...Nh6!? (or on move 9. in the repetition line) instead of 7. ...Qe5, again with good and interesting play.

Thanks!  I usually go with 4...e4 but then I am faced with 5Bxc6 not 5Qe2 to defend the knight.  What is white's most likely continuation in this and blacks replies?

6 Qe2 Qe7 often, you want to play Nf6 and Bc4 to allow you to castle so you need to move the Queen again maybe to e4. So 7. Nd5 Qe4 is a possibility.

I try to castle before taking the pawn back with the bishop.