At higher comp, the assignments are harder, you learn at a different pace from the normal computing class. They scale the mark so it doesn't matter which stream you take (higher/lower), the mark you get from both if you spend the same amount of work would be the same.
The upside of 1711 is the lecturer, more interesting assignments. The upside of lower computing is less work, and this means more time for your other subjects.
by the way, 1711 doesn't assume any prior computing knowledge, so you should really just give it a try, since you don't really know how good you are at computing. I didn't do software design at HSC, and I managed to do rather well.