Don't have a preference for either, ideally you'd like to have a balance between the two, plus both options depend on circumstances to a large degree.
If you have some sort of stability in management, experienced players who can guide the young ones, and the basic framework of the team in place - introducing potential type signings becomes relatively easier because the burden of expectations doesn't always fall directly on their shoulders. Too much pressure for performing at a young age at one of the biggest clubs in the world can often have an adverse effect on a player's long term development. eg. We had Fergie in the dugout, plus the likes of Gary Neville, Paul Scholes, Ryan Giggs, Ruud Van Nistelrooy, Roy Keane when Wayne Rooney or Cristiano Ronaldo were signed. A whole lot of young/ potential type players makes sense in theory, and you hope they'll grow up and shine together, but there's always a decent chance that they'll wilt under pressure/ fail to live up to their potential/ be too inconsistent to win anything of significance in the short of even medium term. Arsenal's teams during the mid-late 2000s and the early 2010s under Wenger (didn't replace the top level experience and leadership ability Vieira, Ljungberg, Pires, Campbell, Lehmann; later Henry) are a decent example of that.
If the team is still very much a work in progress, lacks experience in key areas, doesn't have a lot of accomplished leaders, doesn't really have a backbone to speak of - like the current United team, then starting off by signing a few veterans can have a very positive impact. They can define the blue-print for the other players, come in and start on day one, provide some steadying influence in a transient side (especially when the chips are down), and help the young ones mature at a reasonable rate. A lot of supporters scoff at the idea of signing big name players like Ramos or even Schweinsteiger (still a really good footballer), but often underestimate the importance of leadership, big-time experience, and winning mentality - which is vital for building a successful culture both on and off he football pitch. You look at our team and how many players have been key components in title winning sides ? How many have played at a high level in the latter stages of the European Cup ? How many are the leaders of men that can drag the others with them ? There's a massive vacuum left by the departure of Neville, Scholes, Giggs, Ferdinand, Vidic, Evra and most of all Fergie within the space of 3 seasons; and we haven't adequately compensated for that.
