Harvin isn't near the playmaker that Maclin is....period.
Maclin had less around him than Harvin did and still was unreal.
Harvin is very quick...but not blazing fast like Maclin.
Eh. I don't know if I agree with a word of this. There's a difference between timed speed and track speed, but here are their 40 times from the combine with 10 yard splits:
Jeremy Maclin
40 - 4.45
10 - 1.58
Percy Harvin
40 - 4.39
10 - 1.47
Harvin is faster for the first 10 yards and faster over the total 40 yards. Maclin may have a higher top speed, but most of the routes in the NFL are 5-20 yards. I think it's generally more important to have good acceleration and quickness than it is to have elite 50 yard speed.
As for their production, I don't think it really proves anything. Both of them were impact players. Arguably the offensive centerpiece of their respective teams. Harvin had a good supporting cast, but it's not like Maclin was a one man team. Chase Coffman is an NFL TE, Chase Daniel was a good college QB, and Derrick Washington is a potential NFL RB. Maclin and Harvin both had help. Maclin had gaudier receiving stats, but that was probably a function of his role in the offense and a higher number of targets.
Harvin has a shorter stride than Maclin, which makes me think he has a higher ceiling as a route runner. I think he'll be quicker in and out of breaks, whereas Maclin has a little bit of the long strider disease. He's not nearly as loping as Ted Ginn or Troy Williamson, but he's not as shifty and fluid as Harvin.
I like Maclin well enough and I think he has the potential to be better than Harvin in the NFL, but I don't think he's obviously superior.