What magnusfide said.
Cobb grill- small, with accessories, very versatile, a little slower. No need to have a Dutch oven if weight is a concern.
Seems lighter weight than the volcano, especially if you have to carry a Dutch oven.
Best to cook with lid on, to hold heat in, even when grilling.
Cooks for a smaller crowd. Perfect for 2-4. Can add sides to the moat to cook.
I have 2 of them, with one set of accessories, so I can use them to cook different kinds of things at the same time, or a larger amount of the same thing, if it doesn't require an additional accessory... like 2 fry pans. That is for a larger group. Otherwise we take one and accessories.
Volcano grill- depending on model you buy, it is more versatile as far as fuel source goes.
Bigger food print.
For baking you need the additional heat resistant cloth like lid, unless you are using a Dutch oven with coals on top. Lid virtually eliminates the need for top coals with oven.
Better for straight grilling. Easier to regulate temperature.
Accessories available specific to the grill, but other non specific items work just as well, unlike the Cobb because of it's size and the need for a lid most of the time, for more efficient cooking.
At the present time, Cobb grill accessories are virtually impossible to come by. Two Brothers Grillin', http://www.twobrothersgrillin.com/ have the grills, but no separate accessories. They are working to become the North American Dealers at the present time.
I have both grills because, when they closed them out at the local Camping World, I bought 2 Volcanos with propane attachments for $48 apiece. I also bought the two Cobbs they had for close to the same price, and one they had with a missing rubber foot for even less. I have had the Cobb for a while and the extras are gifts for my children. I am waiting on two extender rings to come back in stock, so they have all of the accessories.
If I had to choose one that would meet needs of camping and home use in the most versatile ways, I would chose the Volcano. If it came right down to it, you could even use it to do canning with the propane attachment.
If I wanted one to cook leisurely, for 2, using charcoal, had limited space, and didn't want to carry a Dutch oven but still bake, I would get the Cobb.
If I wanted the most fuel versatility, hotter grill, and wanted to use the Dutch oven with it, I would get the Volcano with the lid.
With the Cobb, you can always take a Dutch oven too, and just cook with the DO in usual way.
If you think you want the accessories with a Cobb, you would be taking the chance that they might not be available in the US.
Another propane, light weight option, is the CADAC Safari. Also available at the same website. It is a quick and easy. We have used one of those for years. It is what we grab for those impromptu picnics or quick road trips... another close out grill.