@jitb:

According to Glek in NiC Year Book 42, cxd4 and Qb6 are main lines, but Black,facing difficulties, tries now a6... Maybe point of view is a little outdated, I don't know.

Another "new" idea is 7....Be7!?, reference game is Ivanchuk Morozevich,Memorial Donner 1996 and dealt in chesspublishing update  April 2007. I like this idea..because it'is part of Mikhail Gurevich way of play with a later Qa5 ...
Some games: Smeets-Gurevich,Vlissingen 2006, Saric-Gurevich,Cannes 2007, Korneev-Gurevich,2006, Van Delft-Gurevich,Allemagne 2006
I have recorded other interesting games: for example: Svidler-Riazantsev,Moscow 2008

One point is White can deviate from system, trying to short castle with Be2 but here too, Black has good system:  7...Be7!? 8.Qd2 oo 9.Be2 b6!? Nijboer-Narciso Dublan,Barcelone 2007 (Moskalenko)

Sometimes variation can transpose to Nijboer system with Nb3, whatever is move order  (cxd4-Fc5, Be7), because of 
lost tempo when dark square bishop goes to b4 pinning Nc3.

One important idea is: if Black wants to limit White choices to long castle, then 7...cxd4 8.Nxd4 Bc5 is good move order because 9.Be2 Qb6! 9.Na4 (the Boleslasky move against Qb6) Qa5+ 10.c3 Nxd4 11.Bxd4 Bxd4 12.Qxd4 b6  13.Bd1 (to forbid Nc5 because of b4) Qb5! Aseev-Dolmatov,Irkutsck 1986 (Psakhis and Tiemman)

Each of the moves you gave are of equal strength - there are already several books covering the differences between each move. To list some examples: 'The Flexible French', 'How to Play Against 1.e4', 'French Classical', 'Play the French' and 'Mastering the French' to name a few.

I used to like 7...Be7 best due to its flexibility, but nowadays I find 7...a6 and 7...cd4 8.Nd4 Bc5 just as appealing. Especially in the first line, White has failed to demonstrate a route to an edge in recent times. The NIC Yearbooks contained a few good surveys on this line, from what I recall. Particularly the variations within ...a6 where Black plays an early ...g5 offer plenty of fun and also have a stable theoretical reputation. White can avoid such lines (8.Qd2 b5 9.Be2 is one way), but this does not offer White any advantage either.