I always buy joint compound in a box, as it is per unit cheaper than the five gallon bucket. But the key is, never try to work out of the plaster bag in the box. No matter how careful you are, you will have dried fritters getting into the wet stuff and raising heck with your work when you try to spread the mud.
Take the plastic bag out of the box and dump the mud into a five gallon bucket. Squeeze out the remainder from the bag. Now you have bucket mud without paying for the bucket. But, if you are short of buckets, then by all means buy your mud in the bucket container. You never have too many buckets!!! Right?.
Just keep the sides of the bucket wiped clean, and a little water on top when you finish for the day will keep a dried crust from forming. A lid is not a bad idea either.