French or Malgasy?

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?

Official Malagasy is the one that goes in the TV and radio and all Malagasy are supposed to understand it. In the rurual Madagascar you will find few people speaking good French (as the rule of thumb - well educated people speak well french - in the villages they are not many. So it might be wise to stick to Malagasy? However, French numbers are sometimes used even between two Malagasy.
You may want to check out this page: http://dico.malgache.free.fr/ (it's in french though:/)

Thanks.
If only the spelling was not so weird!
I have a phrasebook, but have only the vaguest ideas about how to actually pronounce Malgasy words written in it.

Heh, pronounciation is an important thing, that's why it's usual the first step in learning foreign languages. For me it was very easy, as it seems to be just one difference between Malagasy pronounciation and my mother's language's one ("o" is pronounced as "oo" in English).
Good luck anyway, Malagasy is the most interesting foreign language I've come across. Many words are composed of some other ones. For example "sun" is masoandro, which literally means the "an eye of the day". Sea is ranomasina - "salty water" Logical, isn't it?;)