The MSI and the Asus 570's are an excellent choice, both have excellent cooling.
You will have no problem fitting either one on your Sabertooth as the slots are spaced out well.
The Asus would look awesome with the Sabertooth black shroud I would go with them purely for the aesthetics. But they are larger than the MSI.
Later on you can only SLI with another 570, it can be another brand so thats no issue there.
How much is the 570?
If you can save up some more I suggest a 670 , but it purely depends on your budget.