They're different players, IMO. Atkinson is more of a pure LB. He's allegedly going to be great in coverage, he's a missile towards the ball, and he's a true sideline to sideline guy with amazing instincts. Not that Hill doesn't fit some of that, but Hill is substantially bigger and he's a special pass rusher. Hill's still got work to do in coverage, although it's stretch to imply that he's not already good at it.
Justin Williams? That guy was more of a comp for Hill than Atkinson. He was praised as a OLB blitzer and needed work on reads/awareness/instincts. Atkinson is a pure LB. He can blitz, but he's best in the second level, using what are said to be amazing instincts for reading plays and being a tackling machine. He's a fucking witch on the field already.
Gun to my head, I'm taking Hill over either guy, then Atkinson, then Williams. Hill has the instincts and the elite blitzing talent.
Atkinson is a little shorter than Derrick Johnson was, but they're otherwise the same recruit - pure LBs with blitzing skills, but they're going to clean up and disrupt behind the front 3/4. I'm not saying that Atkinson will be a CFB HoFer or a borderline NFL HoFer, but if you're looking for comps as it relates to Texas, I'd pick Johnson over Hill or Justin Williams. That's just one guy's POV.