I think he chose to hit Viserion because he wasn't looking. Drogon, on the other hand, was aware of the Night King's because he's a potential threat to Danys. Also, any one of the people standing on Drogon's back would have likely jumped in front of the spear knowing how important the dragons are to winning the war. Who knows, maybe Danys could have had Drogon breath fire (or even on his own) in time to melt the spear before it got there. So if I'm the Night King, who we have to assume is fairly smart and clearly has a better throwing arm than Uncle Rico, I try to hit the dragon who isn't paying attention. The one reason he needed a dragon was to break the wall, he didn't need the biggest one, just one. So, I think took the easier shot.

The video you posted is very interesting too because it does appear that his shot was straight on point until the last second. Although Drogon did do a cool twisty thing (that's a technical term) to avoid the spear, but how Drogon knew exactly when to do that is something I can't figure out. Maybe the Night King missed on purpose because he was sending a message, something like "I'm letting you go for now, but I want you to know that I could have killed you." Either way, interesting find man.