How old are they?  If they are over four weeks I'd be happy with either one.  If you look at the feed bag most recommend that you start a Grower anywhere from 4 weeks to 8 weeks, depending in the brand.  In my opinion make the switch any time after 4 weeks that the bag of feed runs out.  Again depending in brand a lot of the grower they recommend you switch to is 16%, though some brands are 18%  Grower. 
I'm a little surprised you are seeing a 16% All Flock, a lot of times that comes in a 20% protein feed though I think it is a different brand called Flock Raiser.  Some people will always recommend you feed a high protein feed, I don't.  Feeding them a 18% or 20% All Flock type of feed with oyster shell on the side once they get to laying age will not hurt them at all, they will do fine on that feed.  I feed a lower protein feed and I like the results I get. 
If they are still growing a higher protein feed will help them grow a little faster and the adults will have a marginally larger body at maturity.  To me that's the only difference in the 16%, 18%, or 20% once they get past 4 weeks of age.