Generally I'd agree with Binka.
But given your location and from my own limited experience of French lakes and rivers, I'd opt for lakes simply because the rivers I fish don't get the attention that they do in the UK. No anglers = no bait going in and in my experience that means difficult fishing for cyprinids such as roach, carp and barbel.
In France, on the public waters, I'd expect that for roach, bream, carp and zander, the lakes will be the best bet. One main reason is that they will be fished, fish will be used to baits and you can more easily discover the hot spots by watching local anglers.