I find the French more aggressive, as black gets easy queenside play with pieces and generally a pawn avalanche. However, my friend debates that the Caro is better than the French because the Caro allows a queenside expansion as well, but "you get to play with all your pieces" (referring, of course, to the bad light squared bishop in the French). We both realize that the French bishop can be traded off in many lines with maneuvers like b6, Ba6, where either white trades the bishop, or has a difficult time castling kingside/coordinating kingside pieces. But its not always possible, and even avid French players will admit that its difficult to activate the slacker on c8. What would be a good argument to counter my friend's argument? Are there any other arguments (from an objective perspective, preferably) to support playing the French or the Caro Kann?
I can think of a couple counter arguments against what your friend says:
1) Sure in the Caro-Kann you can easily develop your Bishop outside the pawn chain, but there are several variations where the Bishop ends up being a target when developed to f5. The most prominent example is in the advance Caro-Kann:
Ultimately, a debate over whether or not the Caro-Kann is better than the French is fruitless. They are both good openings of equal strength, and your choice to play one over the other is a purely stylistic choice.
It comes down to pure taste. I've been playing the Caro for over 20 years now and would never even consider the French, exactly because of this bishop - and because I've played a lot of very nice attacking games against the French with White.
The other reason why I personally dislike the French is White's boring option of 1.e4 e6 2. d4 d5 3.exd5 exd5 4.Bd3 followed by 5.Nf3. (I have to admit I employed this in a few tournament games myself, because a draw was all I needed for my desired overall outcome.) This takes the fun out of any game, at least if Black is a somewhat serious player.
Having said all this, on top I'd like to quote my old coach: "French is an opening for World Champions." What he meant by this is that you must really watch out, because you have to defend against big onrushes in a lot of variations - with a blocked center, the hidden bishop and without "the Black king's best friend" (the knight on f6). It's often a long battle until you can look for advantage on the queenside, because you have to fight for survival on the kingside. Not to my liking, but others feel differently.