I don't know where you got that Cundari had a strong year last year as he was pretty dreadful last season but he did have a pretty good rookie season for the Rivs and has rebounded nicely this season, his 3rd pro season.
I'd say he and Fairchild are pretty even in terms of long-term potential. Cundari looks better today but Fairchild looked better last season (Fairchild is going through a sophomore slump this season like Cundari did last season). If I had to choose a player for a call-up today it would be Cundari. If I had to choose which one is more likely to have the longer/better NHL career when it's all said and done, I choose Fairchild by a hair. Basically, I believe in Fairchild and think he'll bounce back like Cundari has. I don't really see either being true contenders for the top-6 or top-7 for next season though.
Make no mistake though that if the Blues need a d-man from Peoria, it's very likely going to be Jeff Woywitka. Cundari and Fairchild have decent NHL potential (I'd say 3rd pairing potential) but Woywitka's the best option today IMO. The D core in Peoria has been largely unimpressive this season though. Besides Fairchild struggling, Chorney hasn't been doing overly well either (good offensive instincts but sucks defensively...which is sort of important for a d-man), Shields is a decent AHL d-man but I don't really see much of any NHL potential there and Ponich is struggling just to not continually be sent down to the ECHL. I'm pretty sure they called Ponich "Tree" in juniors not because of his height but because he's about as mobile as a tree! Scott Ford and Brent Regner are good AHL vets to have but they don't really have NHL potential. They'd probably be ok as an injury fill-in for a game or two though.