I found stargrass grows too fast and the dwarf sag not fast enough. BUT, I prefer the dwarf sag.
The stargrass tends to grow upwards as well, but you can pinch off the tips and replant them every couple of weeks it grows so fast. You will have plenty of cover quite quickly.
The dwarf sag will take significantly longer to get a decent cover, but once grown it's pretty much care free. The dwarf sag is much better in my opinion, but you can still use the stargrass as well, as a temporary measure to fill in the gaps till the dwarf sag has taken hold properly.