Registering both is better, (.com and .fr both in television ads and in use) but if you have to choose, consider the conditions for .fr :
1. You must have an adress in France
2. There is a big list of forbidden names that can change at any time without any explanation. So your name could be forbidden one day if the AFNIC decide to put it on a blacklist...
3. If you choose a name that doesn't fit their condition, they (AFNIC) can take your name back, but also your pre-registered names, and even put you on a black list and you won't be able to register any .fr anymore... (you can check this in their PDF rules : http://www.afnic.fr/data/chartes/charte-fr-2008-01-01.pdf , 'Article 15, 15.2')
Some consider only the first point... Domainers should read more