I recall reading Ignacio Marin's chess page in the nineties. (No relation to Mihail, I think.) I found it via the web archive.
http://replay.waybackmachine.org/20030604184615/http://cmgm.stanford.edu/~marin/... In his post "Best against 1.e4"
http://replay.waybackmachine.org/20030202024712/http://cmgm.stanford.edu/~marin/... he states
"1...e5. I definitively don't like this. After the most obvious answer, the Spanish, the game tend to be complicated but in general with a persistent white advantage. Please note that 1...e5 has been used by all the LOSERS of world championship matches: it has been one of the main reasons for the defeats of Korchnoi, Karpov, Short and Anand. I think that although in theory black game is perfectly respectable, white play is very comfortable in most lines of the Spanish. Moreover you have to add the Scotch, the clasical Italian/Two knights with d3 and, lately, even the Evans gambit (!). The Russian/Petrov defense can be a solution, but now it is looking good for white. The rest are passive or unsound openings and in general good for white."
Time has moved on a bit from the Kasparov era. I haven't studied any statistics on this, but to me it seems that 1...e5 has become much more popular at top level since Kasparov quit. Top players like Aronian and Kramnik play it regularly.
At club level, I think Giddins discussion about the Spanish main lines in his general repertoire book was good. Once you have the hang of the spanish positions, you can rather easily switch between them. As stated previously, it should be much easier than switching between sicilians.