I've used various aftermarket jockeys ranging from bling to basic. Original shimano ones have a degree of float designed into them which is supposed to aid shifting. Not sure how much difference this makes in reality as I've used sealed alloy ones with no float and never noticed a difference, just mentioning as oe ones will have this if you care.
Main thing is to get ones with the correct tooth count as the originals as there are various out there now.