No no, you're quite incorrect. Both are quite similar in the regard of having mediocre careers. If you take away Del-Shitro USO, both failed to win masters, both only win MM events, both fail against top 3 and both are seen as tier 2 players. David is a better player obviously, with better H2H, better ranking and more consistent game. Del Shitro is just a ballbashing clown whose fans are still hiding behind his limp wrist as an excuse that he can't beat a 31 year old Noserer.
Exactly, if you take away the USO. However, Del Potro did win that US Open beating Federer and Nadal on the way, so there's no reason why it shouldn't be counted. That weekend alone eclipsed Ferrer's entire career, both in terms of level of play and achievements. Even with a limp wrist, JMDP is still easily better than Ferrer.