We faced the same choice and went with the 1150. We are not towing and live in the Pacific Northwest where it can drizzle a lot :). With two dogs the added floor space of the 1150 is very nice. There is also significantly more storage space inside and out. If I remember correctly on the refrigerator side there is a closet, two drawers, and an outside compartment that is very tall. On the sink side you get an extra column of cupboards. (We have a wet bath).
Pay attention to the suggestions on weight. As others have said the 990 is only 330 lbs or so less in weight and these are heavy. Our 14,000 GVWR Ford handles it nicely but that is with the gas engine. The diesel losses almost 1,000 lb's in carrying capacity.