It's better to sign Osimhen than Gyokeres, Gyokeres rarely score against big teams like Dortmund, Porto, and Club Brugge. Osimhen already proven to score against big teams, it will not be wasteful to sign Osimhen rather than trying to gamble with Gyokeres.
Performance of both players in European competitions:
Viktor Gyökeres: 8 games, 6 goals (
European Champions League)
Osimhen: 7 games, 6 goals (
Europa League)
Sporting is not a team that is a candidate to win the European Champions League, so it makes sense that Viktor Gyökeres is limited in the European Champions League, but I think that because he was a Sporting player when Amorin was the coach, it will be easier for Manchester United to sign Viktor Gyökeres, because Osimhen's case is more complex due to the high demands that Napoli is making