I think you need to consider how much of the seating furniture will be placed on the rug. That will change the shape of the rug a bit. If the seating is placed towards the table, then it could possibly still work. The problem is that you are looking at it in a large open space by itself.

A sofa/chairs need to be about 18 inches from the coffee table. All of a sudden the round rug is missing some of its roundness.

You shouldn't think in terms of having the coffee table float like an island in the middle of the room so it isn't accessible sofa or chairs. What's the purpose if you can't put your drink down on it or prop your feet up?

You'll save yourself some more Design Dilemmas if you go with a rectangular rug. Round is more for an entry way or a dining room with a round table.

Unless you live in a yurt.