Macro lens: "extending" versus "internal" focusing
I'm a bit of a novice when it comes to DSLR photography terminology-wise, so pardon the awkward use of language.
I have a Nikon D5000 with a Nikkor 105 mm macro lens which does all of its focusing internally (i.e. the barrel doesn't extend).
I just got a Sony a55 with a Sigma EX 105 mm macro lens that extends like a zoom lens when in use, which I find very odd having been used to the Nikkor.
 is there a Macro lens for the Sony similar to the Nikkor in terms of this behavior.
 is there any advantage of one type of lens architecture over the other?
Thanks for your thoughts!