Hi,
assuming you are going for the premodded repacks, there is basically only one difference between the two (besides the story),
in Sunshine you can create and apply custom overlays to the models.
This should make Sunshine the nobrainer choice for having better caracters, what the game is all about...
The problem is that the the 2 games are not compatible, mods wise, and the "converted mods" may not function properly.
At the moment the mods for the new game are at early stages and may not allow you to use the resources you find, cards etc..., in a proper way since 90% of the content is still created with the old game.
It will eventually be replaced for good, but I don't see that happening in less than 1 year.
Plus there might be a problem with the mods update utility, that screws everything up anytime you do it, but that could be just my issue.
So, if you have to start... I would still go for the old one, when the new reaches a good stage of development you'll be still able to port everything you made