The problem with the old stormies is just that: they're old.
GW sculptors have been getting better over time, which means that they older models look more and more poorly executed the more time passes. This is definitely true of the old stormies with their goofy sized heads and gangley appendages and off-tilt poses. Kasrkin are much better made, which, along with the fact that I use Cadian troops as the base for my army, is why I'd use them.
Plus, apart from the execution, I also don't like the vision of the old stormies. They look LESS armored, rather than much more armored, and their guns look WEAKER than your standard lasgun, rather than stronger. I mean, they probably looked pretty buff back when guardsmen used to look like
this, but really, they haven't aged well in conjunction with everything else.
I mean, it's possible for
GW to do a vac suit properly, but they'd have to make new minis, which I somehow doubt they'll do, given how much they're hyping the old ones.