It's not hard to upkeep a steel bike, even through a salty winter. Frame Saver the innards of frame when new. A good powder coat is good. Steel wool and touch up paint when needed. A steel frame will last with little maintenance. With the abuse a fat bike goes through, I'd be afraid of cracking an aluminum frame -- But that's me pushing the limits of a bike. I've cracked a full suspension aluminum Cannondale, and I've cracked a steel triple butted lugged steel road bike.

That's just my take. I like Surly so I'm stuck with steel. If they made an aluminum frame, I would consider it, if price was tolerable. Who knows. Nothing at all wrong with steel, if upkept and not left to Mother Nature with no oil or paint.