I think the octogon shaped ones give a more realistic catchlight... it's round like the sun. And if you place your lights correctly to get the catchlights in the 11 or 2 o'clock postion, you don't have the white spots in the middle problem. 
Softboxes come in all shapes and sizes... round, square, rectangale, long strips, etc. Each has a place and a reason to use them. Size plays a big part, paticularly in portrait or product photography. You want a size that will cover the area that you are going to photograph. If you are going for soft, even lighting, bigger is better. If you are going for well defined highlights or precise light placement, then a smaller one or a specific shaped one will work better. If the frontal area is big enough and the light powerful enough to evenly light the frontal area of the softbox, it doesn't usually matter if it's round or square or rectangle.
What I've noticed is that you can't get the octogon ones as big as you can the rectangle shaped ones... at least not without paying a lot of money. And the 24" or so octogon won't give you the coverage that a large rectangle or square on will. What ever you get, consider your coverage and make sure you have the light with the power to match the size.
Mike