I personally - and all of this is personal preference - have a MagPul PRS on my 20" rifle. So really what you have to ask yourself is:
a. Do I give a crap what anyone else things about how it looks?
*Cause I just know you'd hate to be ridiculed by the Zombies and/or invading Chinese Jihadi's
b. What are you going to use the rifle for?
* Realistically, the point made earlier about the M4/CAR stocks having a fair amount of play and the potential for causing issues with long range shots in so much that you'll have cheek weld consistency issues.
There are tons of options out there that are specifically resolving the issues with the generic M4/CAR 4-6POS stocks. ACE (SOCOM, AFRX) VLTOR, MagPul (CTR, UBR, M93, PRS) .. Shop around and look through the different pictures found here. Then ask about specific model.