I have an enclosed one for my Abbey. I think she feels safer in it, plus you can unzip the top alittle and she can stick her head out.
My girls are always unzipping it to pet her when we're traveling in the car, but when we're out she is zipped up and looking out the vent sides. Abbey has a pink carrier - I get lots of compliments on how cute it is!
I would say definately the medium. My carrier is 14" long, 11" high and Abbey is only going to be around 4 - 5 lbs full grown. I put a towel and a soft toy in there. They should have enough room to stand up and turn around, I think.