Of what I've understood, you have the Intuos Pro that is more expensive, but has a more genuine feeling - the pen feels a bit heavier, it has more pressure levels etc.
Then you have the plain Intuos, which comes in different bundles (art, draw, photo etc). The difference here is what software it's bundled with. If you already have the art program(s) you need, buy the cheapest bundle (I think draw is the cheapest?), otherwise art or comic seems like nice bundles =) Wacom has a better overview over the bundles here.
And of course, yu have different sizes, I'd definitely go for a medium size, regardless if you chose a Intuos or Intuos Pro =)