The problem with James, as hoops noted, is that if you don't use him as a player to match up with TE's or big WR's (less and less of an issue tbh), he likely won't justify a top-20 pick. He blitzes well, is athletic, and he's good in man coverage. His instincts/break on the ball are alright/good enough, but I don't think they translate to a Safety that makes a significant difference in the passing game. As a matchup piece, he may. But, if your plan is to stick him in 2-deep next to Jones or single-high, he won't return the value.
James never had a year at FSU like Smith had at UGA in 2017. Smith was dominant and all but took away the entire 2nd level. If you're looking to add speed to the D, Smith is much faster relative to most LB's than James is to most Safeties. There were times he looked like he was everywhere. David and Kendricks fell to the 2nd, but both were about 2 years older leaving UCLA than Smith is UGA, and Smith looks like he'll run in the mid-to-low 4.5's (vs 4.65 and 4.61). I'm open to the idea that Miami would view him as too small, but it'd just be another example of Miami being behind the curve. Between James and Smith, I favor the younger guy whose dominant year spearheaded an elite defense.