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.

My questions:

[1] is there a Macro lens for the Sony similar to the Nikkor in terms of this behavior.
[2] is there any advantage of one type of lens architecture over the other?

Thanks for your thoughts!