Originally Posted by john_and_off
the plastic side of the un-53 can be destroyed very easily if you let a ham-fisted shop employee work on your bike
so on second thought, just don't let any inept wrenches touch your bike and get whatever your budget allows.
+1 on this comment.
The plastic non-drive side cup on the 53 blows. That and the drive side is steel, and the spindle is not near as nice as the 73 or 93. On the UN73 both the drive and non-drive side cups are aluminum, the spindle is nicely machined silver hollow cro-mo rather than black pot-metal solid (not hollow) steel on the 53. The bearings however I believe to be the same (at least I could never tell the diff). Lastly, the 73/93 BB shell is cut away a bit to save weight. If it were me I would get the 73 (I have used about 5-6 of them over the years) and they have been without flaw.
The only reason to get the UN-9x over the UN-73 is if you need a 103mm bb spindle. Old Dura Ace was the only Shimano BB that had a spindle that short. The UN-7X never came in any size less than 107 to my knowledge.
Last, not to be a complete self serving a$$ but I have a NOS UN-73 68x107 BB I could sell you if you are interested. Would sell it to you for about the same as you could get the 53 for. I think I may even have new 8mm crank bolts I could throw in.