FA highly recommended
The FA is a great tire, very forgiving over a wide range of conditions. As others have said, climbing traction is excellent, but if you're coming from a Big Betty braking traction might be the more important consideration. In my experience, this is what separates good aggressive trail tires from great ones, and in this regard the Fat Albert is the best I've tried without a significant weight premium. For its size and tread, it's pretty much the lightest out there. The snakeskin is very durable in rocky conditions, and as I said the braking traction over loose is fantastic. It might not be as good as say a Big Betty or a Minion, but those tires are in a different category. It's equivalent traction to a Weirwolf 2.3, which is almost 200g heavier (and noticeably narrower).
Sure, a Nobby Nic will roll better, but in my experience what you give up is the braking traction: again, that's more of a light-duty trail tire in a different category. (I really like the 2.4 NN on the rear, but for rocky & loose conditions the FA is a better performer.)