By the numbers, and realizing that Jackson's are very limited:
Johnson was the better KR. Jackson the better PR. Johnson had a higher YPC, but Jackson has more catches (both are under 1) per game and more touchdowns per game. And, to make it even worse for Johnson, that's with Jackson essentially only playing as an NFL rookie who missed the preseason. Johnson really sucked even more than most people remember, because 3 or 4 plays cloud their judgment. Johnson played 50 NFL games, and had just 39 catches and 4 tds with all that speed (He also had the 2 KRs for touchdowns that people like to talk about). Jackson has played in 14 games, and has 13 catches and 3 touchdowns. If Jackson catches on anywhere this season, even as a 4th or 5th wideout, he could surpass Johnson's career TD and reception totals with just a 27 catch, 2 TD season.
Just as an aside, I really hate the 40 time as measuring stick, because it tells you almost nothing on its own. When you break down the difference in distance it equates to for a 4.3 to a 4.6, for example, it's only about an arm's length. What would be much more helpful to have would be 10 yard splits over the course of about 50-60 yards. That would tell us about initial explosion, acceleration and top speed, and would probably lead to better success with scouting wide receivers. Like the basketball players' "first step", the initial explosion is very often where the battle between WR and CB is won in man-on-man situations.