The two teams you mentioned could indeed be very suitable options for Osimhen, but from my personal view it would be better if Osimhen were sold to PSG rather than Chelsea. Because in the English League it will not be easy for players like Osimhen to compete with other players. Meanwhile, at PSG, Osimhen can play more freely because the pressure in the French League is not that great if we compare it to the pressure in the English League. So the option to PSG seems to be a little better than Chelsea, although both teams are not bad for Osimhen.
You are very correct in the fact that the pressure osimhen may face in the french league may actually be really different a d leaser than that which he will face in the English side as in the french side, there's a legacy of dominance up on which he's just going to establish his composure and reduce pressure on him because it's actually going to look easy somehow for him as a top player in the french top team.
The above benefits in the french league comes with its own disadvantage that the English league will better position osimhen and give him better Chances to be exposed to even more opportunities. With PSG he's got to be quiet limited just like mbappe which has promoted his leave, so if the advancement of the career if of more importance to osimhen, I think the English league should be of priority to him than he would with the french side, I'm very sure even osimhen is aware of all this and knows that the french side may offer more money but he gets more opportunities around the English side.
In the Ligue 1 they don't really get much attention as the premier league does, except for some exceptional performance and players the rest don't get the kind of attention they need to make their career go beyond the level it is at currently. In the English league, they get more attention and have even more opportunities coming in for players, PSG win the league 1 title but it wasn't even celebrated as much as the premier league was so the difference is actually very clear.