Well, what's your goal?
Do you want to get from A to B quickly, or do you just want to have as much fun between A and B as you can? If the former, you really don't want any kind of fat bike, but if the latter any frame material will do. I've had two aluminum fat bikes and I presently have a carbon fat bike. My "near fat," a Surly Troll, is steel. They've all been fun.