Pogba and Marchisio are two diffrent types of player.
Marchisio is very gifted with tactical positionning and cutting out passing and running lines. He pressures the opponent allways at the correct times, but rarely actually makes aggressive attempts at intercepting the ball. When marchisio plays, it allows for Vidal to play a very aggressive pressing game, because He will block off the space in the back or at the side of vidal. It also allows the wingback to make many forward runs, or even pirlo having the occasional forward penetration. When the opponent has a rather defensive central axis and creative wings, Marchisio tends to be allowed to go forward much more, making penetrations in the box. Usually and more recent however, considering the danger vidal creates when moving close to the penaltybox, he makes less of these runs and tends to cover everything. As a result, we are freaking hard to break down from open play, its quite hard to counter us. Even real madrid with the fastest and arguably most talented wide players in the world, struggled to break trough our lines. They never went trough the center, and cross passes to the wide were mostly dealth with.
Basically, because of the work he does, coupled with very complete assets and good passing, marchisio allows his compadre's to play to their potential.
Pogba is diffrent. He too is tactically and positionally gifted. While the disgusting peasant fools were blowing each other over some trickery Pogby showed, i was stunned by the rather excellent positionning he had, beeing 19, having 0 senior experience, in the goddamn serie a ! But he is 19(now 20) offcourse, and that has consequences.
Pogba, allmost like vidal, is a player that loves to go into the duel, be aggressive, attempt anticipation on opponents ball touches and attempt interceptions on that. The problem with that is that he moves out of position often. Not much, but he does, making pirlo or a wide player cover. What he should learn, is when to make those challenges, and when to simply fend off the defender, putting pressure but not going in the tackle,forcing bad passes or moving away from his zone. Learning when to simply hold the midfield, and not hunt alongside. But he can do that. At times with vidal, he clearly showed much potential, where he hermetically closed down the space behind him. Learning to controll space is the hardest thing a midfielder can master. very few are good at it. Its something vidal actually isnt that great at, but he doesnt need to, cause he'll offer the aggression thats mandatory in the central, and players around him, holding the position and keeping the space tight, will force the opponent into duels with vidal, wich vidal likely wins.
In the future, Pogba will grow to have the best of both sides. He'll learn how to controll space, having undoubtedly the talent for it, learn when to hold aggression, but also know when to be aggressive and force the dual. Actually, beeing so good at intercepting allready, he'll evolve to a point where he'll pick his moments to make that anticipating move. Much like cannavaro, who had excellent timing when to burst onto the attacker, and when to hold formation.
Then comes the offence. The big diffrence is that Pogba is much more moving forward. With allready excellent and complete passing, a piledriving longshot, and the ball controll and technique to stay on the ball despite pressure. Pogba is indeed "looking" better offensively, cause he is showcasing the skills he has. However, when Pogba makes his forward runs, it means that a wingback has to hold back, or vidal has to stick as cover. Against better teams, its less ideal, cause one pogba, isnt creating as much danger as a vidal marauding outside the penalty box and licht/caceres+asamoah pressing the flank, with pirlo nearby.
This is however a temporary thing. When he learns how to controll space and his anticipations, its gonna make a diffrence. When his offensive game improves, and oh man how good that will be.., its gonna make a huge diffrence. A diffrence at the point where a forward run of pogba will simply create so much danger, opponents will have to counter it. Basically meaning that eighter you allow pogba to do this thing, wich is bad (for them), or anticipate pogba, wich wont work entirely, but allow wingbacks, and the other cm's to do their thing, wich is just as bad. This will create a situation where we basically have the organisation that we got with marchisio now, but with pogba's excellent interceptions , and the forward attacking flow we currently have with marchisio coupled with still plenty of danger from pogba, or a freeroaming pogba wich will be excellent to watch actually.
Somewere between now and that point, he'll become better for the team then marchisio. When he reaches that point, he quite simply is THE keyplayer. At that point, putting on Marchisio will give the same defence, with less interceptions and offensive danger.
Is pogba allready a keyplayer ? Not if marchisio is on the bench and the other offensive players are in decent form.
Is pogba a keyplayer when marchisio is suspended/injured, or when the other creative players are having a vucinic type bad day ? Absolutely.