The Tester Approves thread (see first post)

couldnt see what justified the lower price on the newer 300

Seems to be down to using third party bearing cartridges rather than Shimano making the bearings. As a result, the axle doesn't need to be hardened and ground to create the inner races.

Reading this, it looks like the shell on the new ones is three pieces instead of one piece with a separate nds cup. This might be quite handy if they’re the same diameter on both sides, because you might be able to use a pair of UN300s and make up a bb with right-hand threads on both sides (e.g. for Russian bikes, XB3 etc).

Previously I’ve done this with Neco bottom brackets because I know they come apart and the cups fit left-to-right.