I'm going to commit heresy and agree with Murf... up to a point.
Let's take a look at two sources that attempt to make independent
assessments of the college prospects -- Pro Football Weekly and Mike Mayock.
Pro Football Weekly Grades
The highest grade given to any prospect was 8.5 while the lowest was 4.1. That's an effective range of 4.4 points. The grades for DeCastro and Zeitler differ by only 0.15 points (6.22 versus 6.07).
The difference between the two guards is only 3% of the effective range. Tiny.
For comparison, the difference between Andrew Luck and RG3 is 41% of the effective range (8.5 versus 6.7).
Mike Mayock's Grades
The effective range is 57 points (from 97 to 40). The difference between DeCastro and Zeitler is 13% of that range (92.5 versus 85). That's still pretty small.
For comparison, Luck and RG3 differ by 3% of the effective range (97 versus 95).
To summarize, the grades for DecCstro and Zeitler are 3-13% apart, based on two reasonably well-respected scouting fan guides. I think you'll find similar results from other sources. Given how inexact the drafting process is, it's probably fair to say that the two guards were "rated about the same".
It's well within the realm of possibility that Zeitler wil make a better pro. But when you consider who they'll be playing for... fat chance of that happening.