In context, yeah it's probably hard to have a better saves per goal ratio playing behind a crappy defense. But the same can be said for Cech. Arsenal's defensive liabilities make Cech look more average than he really is. But crappy defense doesn't play so much into distribution accuracy. Butland is a decent keeper, but I think Arsenal could do better. Especially if we're talking £40 million.
Lets be real here, the only player making Cech look average is Cech himself. He has made more errors for goals than other goal keeper in the league and thats no ones fault but his own.
As for Butland I haven't seen this season so can't really comment on his form however based on what I've seen previously there a certain valuable things he does that I know Cech doesn't. The most notable one being commanding his box, this is an area that Butland has excelled at in the past and gives as much confidence to your defence as good shot stopping does.
Finally with regard to distribution Butland playing for Stoke and there hoof ball tactics will contribute greatly to lower passing accuracy. Is is far easier to pass short then it is to go long. In saying that I think this would be a weakness for Butland given where he has developed would not have put an emphasis on short passing and build play, rather the direct route.