I think they are quite different actually
Diggs has (had) a lot of short area burst and could really get/present open...he really was one of the more QB friendly targets I can remember
Cooper is more your traditional wideout, very strong at catch point, open when he isn't, presents more physicality in his route running and probably better straight line speed than Diggs
In their prime I would say Diggs was perfect for a developing Allen, Cooper is perfect for a developed Allen...granted neither are in their prime anymore but no fixing that