This depends on what you feel about the potential Bg5 pin. I particularly don't like the pin, so e6 first allows me the option to follow up with Be7. BUT If I play Nc6 first -- a good developing move -- instead of e6 then I can give my own pin with Bg4 if White develops his knight (Nf6) next, since I haven't blocked in my bishop. (Notably, the blocking of this bishop is a common theme, and biggest problem for players of the French defense and other early-e6 openings).
In your first line, Black locks in his bad bishop in order to cope with White's pin. In your second line, Black would rather get his bad bishop out of there than locking it in.
So, if you were deciding between both, you would want line #1 if White's Bg5 pin is difficult for you. And you would want line #2 if it is more important to develop your bad bishop. (How difficult is it for your game when it is locked in?)
I'm about to begin a Caro Kann tournament and I'd just love some brief input on the difference between
4.c4 Nf6 5.Nc3 e6 and 4.c4 Nf6 5.Nc3 Nc6
I'd appreciate some input on the implications of the two different choices by Black on move 5. What it means for the game (if anything), what one solves versus the other, and what one should be on the lookout for in each case? Thanks.