I'd estimate myself at 1500-1600 USCF.  I recently stumbled across the Gajewski variation of the Ruy Lopez, and I'm excited - it suits my playstyle very well.  I've also heard good things about the Marshall, though.

-Can anyone offer thoughts on pros v. cons of the two variations, both in terms of theoretical soundness and practice at my level?

-http://www.amazon.com/Attacking-Spanish-Marshall-Schliemann-Gajewski/dp/1906552177 - worth buying?

-Besides the Schliemann (which I don't like much), are there any other tactical counterattacks for Black in the Ruy that I should investigate?

-Finally, what (if any) options are there for White to deviate from the Chigorin mainline and steer the game into tactical positions?

Thanks!

Dont play the marshall, white has many antimarshals at his disposal, and what is more, he can easily force a drawish endgame (marshall is nowadays used as a drawing weapon as black! sounds funny but it is true)

Try archangel defense instead, it is very tactical and complex, probably sth you are looking for, u could also play Bc5 instead of Bb7, but then you have to be ready to sacrifice pawn in 1 variation, where white plays c3 and a4 and b5pawn will be lost