Aluminum is your friend. Really.
First, I strongly advise aluminum wire such as MH feeder. Copper just doesn't make sense on heavy feeder. Aluminum feeder has always been reliable (unlike what you may have heard about small branch circuits) and the lugs on the panels are aluminum anyway. Why make a dissimilar metal joint?
And if you're thinking "Well ...
UF-B is rated for direct burial, though you'd be better served by running through conduit for a couple of reasons.
Conduit doesn't have to be buried as deeply
If When the cable fails, conduit makes it very easy to pull a replacement instead of digging up the old cable and laying new wire.
Conduit will allow you to easily upgrade the wire size (assuming you'...