Rocker.
I think Rocker had a shaky outing on national TV and had some velocity issues this year - that were seemingly a blip on the radar - and now his perceived stock is somewhat down among fans and pundits. Rocker will have a great pro career. The percentage of outs Rocker gets with his offspeed pitches suggests to me there is a chance he will adjust to pitching out of a professional rotation relatively quickly. In mid-May according to fangraphs over 70% of his career K's in college have come on curves. Facing professional line-ups multiple times in the same game requires more than an elite fastball. Rocker has the depth of repertoire to make the jump and his fastball isn't far behind where Leiter's is.
The improvement in his overall stat line from his freshman year to now has also been impressive:
1.094 WHIP, 7.9 H/9, 1.9 BB/9, 10.3 K/9 and a 3.25 ERA in 99 IP his freshman year vs. 0.934 WHIP, 5.5 H/9, 2.9 BB/9, 13.2 K/9 and a 2.73 ERA in 122 IP this season.
The statistical improvement, the overall D1 control related numbers (4.72 K:BB, 2.6 BB/9), and the physicality really make me think he a has higher ceiling compared to any other notable pitcher in the draft - even if his floor isn't as high as Leiter, arguably. Build-wise he looks a lot like Strasburg, he's 6-5 and 240+ with tree trunk legs. He just looks like a guy who has 200+ IP written all over him. Aaron Nola is the only college pitcher in the last decade that jumps to mind as a better prospect going into the draft.
That said, Detroit will never let him pass them by. I think it's much more likely that Leiter is on the board at 4 and Rocker is not, but I'm just guessing. I think the most exciting thing for a Red Sox fan would be if Leiter and Rocker were both available and Chaim has his pick of the Vanderbilt litter.
E: a couple words and a couple punctuation errors.