I don't speak either.
If I am prepared to do some learning, which of the two would be more useful throughout rural Madagascar?
The obvious answer of course would seem Malgasy (also easier to learn grammarwise), but books only teach the official dialect, and I am not sure quite how much that is spoken away from the highlands, given that differences between dialects seem quite marked, and LP even claims standard Malgasy is resisted by many in coastal areas.
French, OTOH would be pretty much the same throughout the country.
So would the average local out in the sticks be more likely to speak and understand French or the standard version of Malgasy?
