This is a trickier one than the last.
Superficially, the answer is "Venice, obviously" - it doesn't need to be allies, and Austria often has a tricky decision to make: because you need alliance, marrying a CS sacrifices CS benefits you're already getting, which are often more valuable. This makes Austria far more interesting than Venice to play, but potentially somewhat less powerful.
Then again, Austria can annex CSes - Venice can't, and so will never gain the happiness advantages of courthouses or the flexibility of manual production (though since many players never annex, they won't notice this advantage). Gold in quantity is easier and quicker to come by than Great Merchants, while Venice is reliant on key Wonders (most of which are however not hard to get, except Colossus). Austria also isn't restricted to only CSes - it can expand normally as well as selecting key CSes (but since so many players favour playing tall, this is also less of an advantage than it looks). Plus, due to the Patronage finisher, Austria can get MoVs - Venice can never get diplomatic marriage.
On paper, Austria looks as though it should be stronger. In practice I find Venice so powerful that it's very boring to play - I was running ahead so fast that I didn't bother to complete my first couple of games with the civ. However, that's due mostly to its doubled trade routes, not CSes.
Venice is the stronger overall civ, I would say, but they are certainly not better at getting CS's. In fact, they aren't made to be better, either. They are meant to get a few nearby CS's. Austria is made to be able to buy them all up.
I actually voted Venice overall, considering the civ as a whole, but the above sums it up exactly - if just considering "which is better for playing with CSes", it's Austria.