There are also some interesting things to add. You mentioned how Unga was a RS freshman. Jamaal was a barely 17 yr old true freshman. And he didn't start the season either. Nothing against Alisa but breaking his arm was a blessing in disguise because Williams probably could have started the season.
Over the first two years, Unga had over 100 more carries than Jamaal did. That really helps make the difference in yards. I saw an argument above about Hill making it easier, but Hill also had 250 carries and Williams only got 217. His total yards and TDs would look a lot different if Hill and Anae would have given it to him more and he was actually used in the RZ.
I don't like comparing the two. But sophomore to sophomore, I think it is silly to suggest he "isn't at the same level" at the same point in careers... And this is with Williams missing two games, and Unga missing 1. And it's also fairly safe to say that Jamaal's jr year will easily trump Unga's.... And for once, a similar schedule. Unga did have a great supporting cast and a great schedule to play against.
I hated thinking about what things would be like after Brown left... Then I felt the same way after Unga. I was going to miss him. Now I REALLY feel that way with Jamaal. No matter who is better than who, I hope BYU keeps landing guys like this.