Some people choose to construct their BBQ Islands out of cinder blocks and others use steel studs. Both have their merits. If your island has curves in it then cinder block is the way to go. If you are trying to maximize internal storage space, steel studs are the best choice. The main thing to remember is that the spacing of your components is critical to your BBQ island's usefulness. As long as you are purchasing medium to high end cooking components, the spacing of the equipment is probably more important than the actual components. If you can't set a plate down next to your grill, you will be unhappy with the finished product.
Increasing the width of your BBQ island to 10' gives you some options that you simply don't have with smaller units. By adding another 2' to an 8' island you can add another surface component. The most popular choices are sinks or trash receptacles. Most people prefer the sink option with a slide out trash can under the side burner. Having a sink really comes in handy, you can wash your hands and BBQ tools without ever leaving the party. This is also the size where you get to start thinking of the fun stuff to add such as kegerators and beverage centers. Some people do built in pizza ovens and smokers if you are into either of those. Plenty of options!