[SexyJin]

Hmm...thats tough. I think quadruped would be...uninteresting? I mean when you read a story about a girl that can squeeze through a tennis ball sized hole, running on four legs just doesn't seem as interesting. That and it seems the Body's natural form has to do with tentacles so it would seem more likely to have a beast with 8 or more legs than just 4.

Avian on the other hand would be interesting for both the reader and for Mary. It's always been a human dream to fly without machines, so I could easily imagine Mary finding air travel a much more fun and safe way to get around. And this option does allow for maybe some more interesting events.

as Flango said "but then she flies too high and accidently hits a helicopter or plane, and The Body takes over and eats it, Mary inside screaming."

Or maybe she's hit and then gets splats onto the ground below, sending pieces of her flying all over, forcing her to put her self back together. It could either be all at once, or she could go around looking for individual bits and tentacles of herself as she's half formed.

So basically I vote Avian.