Both have been to 4 in the world, both have won a lot of small tier events but JMDP has a US Open title so his career is quite comfortably superior to Ferrer's. That being said, Ferrer has things in his game that bother the Argentine such as the ability to rush him (JMDP likes to get a rhythm going off the ground but Ferrer gets balls back with interest)
Pretty much this. It's not like the Murray vs JMDP comparison before Murray won the USO either, because with Murray it was always obvious he had the potential/ability to win a Slam himself too, which Ferrer simply doesn't.
And yeah, it's an awkward matchup for Delpo. He hates playing guys who can absorb and redirect power seamlessly, because his default gameplan doesn't work against them, he needs to be more offensive. Against most players he can gain the upperhand in a rally by hitting one single forehand (even if down the middle), that is not the case against Ferrer (and the current top 4). Still don't think it will that much of an issue if/when JMDP gets himself back to top form (no injuries), the gulf in ability is big enough to make matchup issues not as relevant, although Ferrer will probably remain someone JMDP will not want to see in his draws.