Mando has 4 tones (5 if you count the one without any accents) and apparently Canto has 8 (i'm told). The problem is that mando and canto tones don't overlap, so if you know both languages, it's 12 tones you can distinguish from. The other problem is that Canto is much more colloquial. There are a lot of shortcuts/slangs that you don't see in formal writing. That being said, the key is to learn written Chinese. Mando sticks to it a lot more, making it easier to learn (and less tones also helps). In canto, written and spoken are very different.
Luckily, my parents had the foresight to make me learn mandarin to pick up the written language while i grew up in a western city. At home, of course, i would speak canto to them. Oh and yes, French is useless, i ditched it for mandarin when i got the chance

i wish my mandarin is better though, i'd like it to be at college level.