I ran these two videos repeatedly about 20 times, and I can see your apprehension - although I feel that you are a little too critical - which is just fine when you are selecting a herd bull. You mentioned that the stiffness is not as remarkable in person as on the videos. It could be that when the videos were made both of the bulls had been a little stressed, or had been lying down for some time prior to the videos being taken - or - or - or - - -possibly a dozen different possibilities could have been the cause. It strikes me that neither one of the bulls have had enough walking exercise on a daily basis in their lives! In any case, you might mention your concern to the owner(s) and see what their thoughts may be. They are full brothers (flush mates??) so a good look at the sire and dam might be in order.
Phenotypically speaking, I prefer Lot 4 to Lot 5, but only slightly. Lot 5 is sickle hocked, but not appreciably. They both have a lot going for them. The prime factor here, of course, is how they will nick, or balance, with your cow herd! Their genes are going to dominate your entire herd for many years if you retain heifers from them.
DOC HARRIS