Are there certain applications to use MC rather than MMF, and vice-versa? It seems like both can be used to create 3D figures and I'm not sure which would be better.

I'm still rather new at all this and have only used MMF as cut out pieces on a BC cake. I have NEVER worked with MC but would really like to give it a try. Thanks!

Yes! I use modeling chocolate instead of fondant for everything, especially for modeling figurines.

You can also make simple flowers that get set faster in the fridge with modeling chocolate, as opposed to waiting for gumpaste/fondant to dry for hours, if you're in a hurry.

http://jessicakesblog.blogspot.com/2012/06/ruffles-and-lesson-learned.html

You can also mold modeling chocolate better, especially over Rice Krispy treats, to smooth out the bumps.

http://sugarsweetcakesandtreats.blogspot.com/2010/11/toy-story-cake-and-cupcakes.html

And I think it's more pliable than regular fondant, smoother figures, etc...and as they say, more delicious in case you eat the figurine.

http://sugarsweetcakesandtreats.blogspot.com/2011/08/giraffe-themed-first-birthday-cake.html

In any case, there's a lot to be learned at this blog, I think.

this may seem like such a stupid question and i do apologise in advance lol.............. but with modelling chocolate, how do you colour it???  I've always wanted to try it but in my head i've only got the colours cream from the white choc and brown chocolate!
 

ahh I see!! Thank you for clearing that up for me, I always wondered how it was done ;)  I might have a go using the white candy melts and adding colour to the corn syrup! 
 

Thanks everyone! Which one holds up better in the heat? humidity?

Fondant works better in the heat and humidity.

I like MC because it doesn't dry out, it simply hardens.  My climate, however, is only extremely hot and humid a few weeks each year.

I massively prefer MC for everything... although I do panic slightly when the cake has to sit by a window or ina hot room!!!