Both those things are evident, Rafael has improved, but his mistakes are usually huge, massively out of position rather than a little. De Gea is quality, the best keepers coming out to get crosses, not always, but are usually weaker at other parts of their game.
Coming to get crosses isn't a de facto part of keeping, some keepers do it, others don't, some are great at it, others aren't.
Defences play in different ways, people have a go at Given for not coming out for crosses, but if you have defenders who are better than the keeper at winning crosses, why on earth would you put the keeper out of position when the CB has a better chance.
I said earlier that an established back four does more for a defence than any individual component, look at Newcastle last year, Santon isn't particularly good I don't think, Simpson is barely average, Colo is half decent but not brilliant IMHO and Taylor(s) certainly aren't great, but having the same 4, a keeper who knows exactly how the back four will react, all adds up to a solid defence.
Arsenal, champs league defensive record for lack of conceding with Cygan, Senderos, Flamini and I always forget, but think it was Lauren.... the main thing was because of injury these guys played basically every game for a while and worked as a unit brilliantly, while most people think Cygan and Senderos are crap.
For me Vidic is both, highly over rated but most importantly, a bit of a sick note now, Ferdy has had a very poor fitness record over the past few years. If you want De Gea to improve... its by selling Vidic while he has ANY value left(maybe get 5mil now, in 3 years he'll be worth nothing), Ferdy, already worth nothing, and buy two new CB's.
Jones both hasn't forfilled his promise, and Smalling looks better but the biggest problem, is both are constantly injured also. Two average CB's who can play 60 games a season will do more for De Gea than anything else. If Utd fans think a new keeper, playing with a constantly changing defence will look better, they're in for a shock.
Settled defence is the key to both Utd's conceding problem this year, and De Gea's "mistakes", replacing the keeper would leave the same problem and another keeper fans would tear down for no reason at all.