I am currently multi-tasking between two seperate games of Divinity Original Sin, one solo and one co-op with my daughter. It is exactly the same game played on exactly the same computer and yet it feels like two completely different experiences. (Side note: Divinity allows split screen local co-op on one PC (couch co-op) as long as you have two game controllers. It would be nice if one player could use keyboard and mouse but it is still pretty sweet).
My solo game is slow and thoughful. I regularly spend half an hour in the inventory screen sorting gear and comparing stats. During combat I ponder skills and tactics carefully before making a move and sometimes I go back and re fight battles I have already won just to see how a different strategy might work.
In the co-op game momentum is everything. You can't spend long browsing your inventory if your partner is barelling along to the next encounter. Each new area is looted at twice the speed and snap decisons are made about the best home for equipment: "Want some poison arrows?", "Is +24 armour any good to you?"
At least the turn based nature of combat gives us a chance to discuss tactics and act in a co-ordinated fashion. After all we are sitting right beside one another so communication is not a problem. Nevertheless decisions are made and acted upon far more quickly than when I play solo and fewer alternatives are discussed or tried. We never go back and replay an encounter just to see "What would happen if ... ?"
My personal campaign has advanced far beyond the co-op game so my familiarity with the game does speed things up a bit: "Talk to this guy. He has a quest". Nevertheless we have chosen different skill sets and we are using a different control scheme (controllers instead of mouse and keyboard) so there is still a lot of new stuff to figure out as we play.
I would hesistate to say which playstyle is better. The social dimension of the co-op game is special but I defintely get more engrossed when I play on my own. One game, one computer. Two completely different gaming experiences.