I have found that French oak does not have the degree of oak flavor that American oak has. If you use heavy toast, American can be over powering. I prefer the house toast of Hungarian which I find is between the two.
French is more subtle and to me takes a bit longer than American. I usually use American early (primary and secondary), Hungarian cubes mid term (clearing) and French later (in aging). I don't usually use all three in one kit but that's not to say that I haven't either. This link gives some terrific oak descriptions of the effects of oaking. http://www.xtraoak.com/