I think if the bearings were to be dissembled both would turn out to be angular contact. If for no other reason that a headset is a REAL bad application for a radial contact bearing (as much of the loads are nearly in line with the steerer).

A simple measurement or comparison of the specs would likely answer your question as to the interchangeability between the two types. I suspect that what you see are the two differing manufacturing designs of the same bearing spec. (Assuming as Slash5 said, that the bearing seat angles and steerer diameter are the same.)

BTW it's pretty easy to pull off most seals from these types of bearings and flush then replenish the grease. We do this often at work when we don't have the correct replacement on hand and the customer wants his bike back ASAP. Andy.