What separates Earl Thomas from FS Reggie Nelson?

[cland]

I'm looking at the guys' prospect profiles, and some college highlight tape. They seem to be regarded in the same way as a FS/CB due to size concerns, but both have pulled the typical "the next Ed Reed" comparison. Now Reggie Nelson slipped to the Jaguars at pick #21, but has to be considered a bust at this point.

Now I know we don't want to write off all undersized safeties due to one bust, but after watching some of Thomas's suspect tackling in a highlight/lowlight video I'm wondering how he compares.