I think that ultimately Macheda will have more of a chance to succeed at OT than Welbeck. I think Macheda has more technical ability, and Welbeck will just never get the chance. I rate them both, Welbeck is a great finisher, but I just don't see BOTH of them getting a chance at OT soon enough for them to want to wait around.
Welbeck and Macheda are 20 and 19, respectively. That's still very young for players at a club that is the size of Manchester United, and particularly in a role that is as important as striker/forward.
The club appears to be following strategy in order to develop players with a view to them becoming genuine first team members as late as 21-23 years of age, at which point performances become indistinguishable from the rest of the first team, both in terms of consistency and maturity (see Rooney and Ronaldo, for example, but also Beckham, Scholes, etc).
That obviously doesn't mean that they won't be given any game time before that point, only that the evidence overwhelmingly suggests that it is at that point that players start to mature. While 21-23 years of age may seem too old, particularly as other clubs use players at a much younger age, a club as successful as United can't really afford to develop too many players within the first team squad.
If a player is going to make a meaningful contribution, then their performances need to be at a similar level to that of their peers, particularly if they are to be trusted in the number (and importance) of games that would be required to develop them in the first team.
There does appear to be a difference between foreign born and British/Irish born players, and also players that have come from the academy and those that are bought from other clubs. The club doesn't appear to want to loan out foreign players and/or players that are bought from other clubs. They need time to develop and understand the demands of playing for Manchester United, which is obviously something that requires them being around the first team squad.
I also disagree with your assessment of Welbeck and Macheda. Both are very different players. Macheda is probably the more natural finisher, whereas Welbeck is much better technically (as 'technique' involves every touch of the ball, although it is a rather vague term). Macheda is more suited to the number nine role, whereas Welbeck is very much a player that drops off in to the hole.
Both players have a good chance, in my opinion, as in a years time it is conceivable that both Owen and Berbatov will no longer be at the club.