It depends on how you define athleticism. Neither of these two guys are quick, agile, and neither one of them is really a high-flying basketball player. Booker probably has a slight edge, but not much of it. Both of them are good ball handlers, crafty, have great balance, and both of them are extremely smart. Luka definitely has a slight edge there, mostly because he's played an actual point guard professionally for five or six years. Luka is also a LOT bigger than Book. That makes it easier for him to see over the defense and make better passes. He can also hold his own in the low post better defensively, and it allows him to be able to "bully" people on offense. Flip side of that is that someone who is two or three inches taller and 25-30lb heavier is going to be a little slower. Book is probably a better shooter, and both of them are quite nasty and both of them have "it." The main point here is that athleticism is not just about how high someone can jump and how quickly they can change directions. Frankly, talking about those things as they relate to Luka and Devin is like talking about speed-racing between a Toyota Corolla and a Chevy Cavalier. Neither one of them are going to be winning anything based off of their speed, quickness, agility, and the jumping ability.
All in all, all things being pretty equal, I'll take a point guard built like a power forward over a prototypical swingman, especially if he happens to be younger. They are both excellent players with incredible skills. They do most things similarly well, and neither is perfect. Both of them are among my favorite players in the league, but I do give Luka a slight edge due to his size and versatility. I just love the fact that he can be a point guard on offense and a power forward on defense, even though Booker is clearly a more capable perimeter defender.
EDIT:
Another big factor for me is that Luka seem to be able to see things develop better and faster than Book. Again, this is likely due to him having played PG for years. I am not quite sure if I am explaining this well, but he just seems to understand how plays develop. Booker can make a pass, but Luka seems to be able to see what pass he needs to make much faster, and he understand that he has to make a pass to player A so the player A can set up player B. I don't think Booker sees plays ahead of time the way Luka does. Note that this is not a slam on Book at all. I honestly haven't seen anyone who can do this better than Doncic since Steve Nash in his prime. Before Nash, you'd probably have to go back to Stockton or Magic, though Stockton was more of a pick and roll master/ specialist.
