Parker is a gamble on a player with upside. Still is what he is, which is a deep threat that contributes every now and then.
Though it seems like so long ago, there were times like in NE four years ago when Parker was so unstoppable New England resorting to holding him on deep balls to avoid getting beat on yet another catch. There's a level of POTENTIAL that Parker has that Stills just doesn't. On top of that, we can at least attribute Parker's struggles to injuries. Granted, that's a skill too, but Still has disappeared all the time despite being on the field.