Oven size really comes down to what size group you will be cooking for. If you are going to be very much cooking for 2-4 people the 8" size will be handy. You can scale down a cobbler recipe for an 8" oven and it will be a fine serving size for 4, and probably too much for 2 people. Of course, I'm assuming you are planning to serve the cobbler accompanied with ice cream - you do have a freezer right?
I also have a 10" oven. Most recipes sized for the 10" oven make a little too much food for 4, and are about right for 6 people, especially if it's being served with other side dishes. For example, I can make a mountain man breakfast in my 10" oven and feed 6 with leftovers.
I haven't gotten into roasts and such in a dutch oven as I usually don't take that much time in camp for cooking. As others have suggested the larger oven (probably 12" deep) would be best for that type of cooking.
I have the nylon zippered bags that Lodge sells for each of my ovens. I like them just as a convenient way to transport the oven while keeping it clean, and once it's been used to keep the ash and soot deposits on the outside of the oven off the rest of my gear.