Oooh! Which one did you choose? I have the Fuchsia Snake mini MAC with silver hardware and I think it is just so pretty!!! So with me, I'd definitely go for the Aqua.
I was actually looking for an Aqua one before, couldn't find it but found the Fuchsia one on sale over at ShopBop (shipping was friggin' fast and packaging was great! They were terrific). The Snake embossing on it just makes it so much more interesting in my opinion - gives it a little more "look-at-my-beautiful-bag" factor! Plus, I'm more partial to silver or light gold.
I can actually use my Fuchsia Snake Mini MAC for both casual and formal occasions - but of course, with the "formal" event, that would really depend on what kind of environment or culture you have where you are. Where I am, snake embossing can actually seem a little formal...at least in my mind. Lol. Really though, the Snake Mini MACs are just so pretty.