I actually think Diaz and Foden are quite similar from what I've seen, similar in stature, similar in their moment and control of the ball... there's no need to get defensive of one or the other as they are both supremely talented and they are both on our books.
Foden looks like he could have come from Spain when he's out on the pitch, which is maybe why some place him above Diaz because you don't expect an English lad to be anywhere near as technical as someone like Diaz in his age group, it very much looks as though he is though. This is a good thing IMO, there doesn't seem to be anything one can do that the other couldn't if they put the work in. If they are the same age and kept together they could hopefully push each other into becoming two very special players indeed who are more than capable of challenging for a first team place.
Their goals against Stoke were so similar, Diaz showed an intelligent knack of placing his body between defender and ball at all times along with good strength to outfox several defenders(a rare knack to have, he must be watching David Silva), Foden beat the same number of people with cleaver movement and changes of pace with a nice feint to finish the run in a perfect position to shoot from(arguably the cleaner setup for the cleaner finish). I don't think there's much point in arguing over who's better they are both bloody good and there doesn't seem to be much in it either way.