If Osimhen were so in demand, then some team would have bought him long ago, but he continues to play in Napoli, and all his possible transfers end only in rumors. Sooner or later PSG will win the Champions League, and Ligue 1 remains one of the top leagues that provides many opportunities for the player. Mbappe, by the way, came to PSG from Monaco, and now he is a Real Madrid player, in my opinion this is a very good career development, and it didn’t hurt that he started in Ligue 1.
As far as I know, last season Osimhen had a very large transfer market value which made many clubs unable to get him. Napoli also won't let other clubs get it easily. Because of that, they set a high price clause for Osimhen. This season Napoli's performance has been very bad. Osimhen managed to score 15 goals and was among the top scorers but was unable to provide a good performance for Napoli.
I think if given the choice in my opinion PSG is better than Napoli. They can almost always win the league so they will always be able to play in the UCL almost every season. But the problem is that currently PSG is not a good choice if you want to have a career. They are a good team but Ligue 1 is not as competitive as some of Europe's other top leagues.