Josh Primo is a better athlete
Coming out of Kentucky, there was no doubt that Devin Booker was a highly-skilled player and one of the best true shooters from his class. The main concern about his game as he started his NBA career, though, was that he was not quick or strong enough to create space on his own.
Of course, obviously, Booker proved this to all be false. He is an adequate NBA athlete and is more than crafty enough to make up for any athletic shortcomings. While some have had similar concerns about Primo, I don't think this will be an issue for him. The Spurs have faced a lot of criticism for taking him so early, but the one thing most agree on at this point is that he’s at least a good athlete.
Primo is quick on his feet, deceivingly strong, and has a great leaping ability. His 37.5-inch vertical jump places him significantly above average. In fact, that's only about 2.5 inches short of Lonnie Walker's maximum vertical jump measurement.
As a secondary ball-handler, like Booker, Primo also has the craftiness needed to create space for his own shots. He's already proven himself to be a good shooter off of catch-and-shoot opportunities and has shown evidence of having a decent pull-up jumper and dribble-drive game.
When accounting for his athleticism along with these skills, though, he could become a better shot-creator than Booker. While Booker’s average athleticism has obviously not stopped him from becoming an elite player, in this way, Primo already has him beat.
Statistically, their college careers were very similar, so just the fact alone that Primo clearly excels as an athlete is enough to make Spurs fans excited. He could theoretically evolve into a top-tier NBA three-level scorer, and even if it takes some time for that to happen, it would be well worth the wait. Even beyond this potential, though, Primo offers even more that could give him a leg up on Booker coming into the league.