McCoy suffered his concussion in Week 11.  He didn’t play again until Week 16.  As far as concussions go, that’s a fairly long period of time and reports throughout were not all that encouraging (and for a McCoy owner like me, downright scary).  He’s back now and in the world of dynasty, he gets a free pass (for which I am thankful).  But he’ll only get this one.  Another concussion like that would crush his value imo.  I’d much rather invest in a comparable asset that still has their free pass.
As for which asset is safer, my use of “safe” in this context is market value safety.  It sounds like you are talking more about production safety.  Two different ways of looking at the situation (although my view of Wilson’s production safety is probably quite different than yours as well).
As far as market value goes, David Wilson needs to do far less over the next two years to increase his value to McCoy’s level and beyond than McCoy does just to maintain his.  It is entirely possible that McCoy will significantly outscore Wilson over the next year but still have a consensus market value at that time that is less than Wilson’s.  The way I play dynasty, future market value and market safety are quite important.
There is a cost to that market safety, and that is expected current production.  I think the cost in this case is worth it, but I can understand those that do not.  Throw in a mid-1st with Wilson in order to adjust for current market value and you’ve got a really good deal imo.
		
		
	 
I have to question your stance on each player's market value.  It's quite an anomoly for a player to be benched and in the doghouse for much of the season, and see his value rise.  I don't expect that to hold as the norm, myself.  He's two fumbles away from being right back in the doghouse, and he has a history of fumbling.  He has work to do in pass protection and he won't be the starter until he's much better in that area.  What happens to his market value if he finishes as RB 20+ and Brown finishes as RB 17 or so?  Right now, he's not the 3rd down back and he's not the goal line back.  Is 2 down work without GL work enough to keep his market value where it is, assuming he can get 60% of those carries, which isn't a given in my mind?  He's not going to get the receptions to keep him floating in PPR leagues.
This dynasty RB class is very weak and old and I think Wilson is being valued higher because of that.  He's a 22 YO 1st round pick with a solid shot at a solid job.  That's rare right now.  But next year it might not be.  This years top 4 rookie RBs all have opportunity to shoot up rankings, and next years class will very likely provide another couple young, starting, taletned, 1st round backs too.
Go back 2-3 years and look at dynasty rankings - compared to today, we were spoiled.  AP,CJ,Forte, MJD, Charles,Rice, McCoy, Stewart, Mendhenall, McFadden, etc.  Plenty of 22-25 YO elite talents.  Place Wilson in that group and his value plummets.  He is rare right now, but might not be, even this time next year.  If two of Lacy, Gio, Ball, Bell pan out, and next year provides 2 Wilson level options - he's now one of a sizable group who are young and very talented.  On top of that, guys like Pead, Lattimore, Miller, etc, are young and have a shot to be worth a lot this time next year.
I don't think the market will be as kind to him as it was this year, without more reason to be so.  Just my thoughts.