I would sell him for 25M. I'm nervous about Pogba's ambitions and I'd rather him be a first choice player right away to calm his possible intentions to leave.
Plus, he is miles ahead of where Marchisio was at that age. It's almost certain that he'll end up better than Marchisio, if he isn't already.
Marchisio's value will never be higher and he's likely peaked (although that's not certain of course). So if we were ever to sell him, it should be now, especially with a direct replacement already on our squad and the possibility to use that money for a more pressing need (attackers).
Basically it comes down to whether you want to sell him at all or never. Because if you want to sell, really it should be now.