Solo you take all results. Co-öp you take your turn. Whether I roll or my buddy rolls the combat die will change the victim of damage. Either will remove whole regiments, so number of turns in combat is about the same.
The XPs are being divided co-öp, and XPs mean level bumps for more HPs. Surviving combat with damage gives a better chance of using Trauma Kit and getting points back. Can't use "Heal" inside of corps battle. Four level ones have more HPs (in total) but anyone might die on regiment return fire (damage). Getting all four of a group to even level two is not guaranteed before a character death. Solo you hit level two (and up) rather fast. Solo player to level three will take eight XP. All go right to you. Four level one all up to level two needs minimum of twelve. That never seems to spread evenly.
Some ways the experiences of the game differs solo and co-öp. I do die more often at get-go solo if I start with some poor rolls. Need that level jump to feel okay. The +3/+3 defenses do make that less likely after I rout some enemy regiments. Safeguards also block the damage, but you do regress and lose value in your Seabag stuff. Then poses the challenge -- go attack something looking to try to replenish Seabag while company as a whole is suffering? Group play someone is typically pristine (dumb luck) when Time Lord is faced or a enemy command (nasty negative to a single D6).
Knife and Pistol make good defense against loss to Seabag ratings. When given a choice of METT-TC it's tough to know what to get. Smoke is nice to drop Threat. Big guns are nice to just win the day fast.
Poor start of combat rolls also might kill members of the group. Remains a problem longer than solo who jump up to 20 or 36 HPs. Loss of Medical UGV may be game over, if you don't know who is Time Lord by then.
Both play methods are losing marines in the company. Yes, METT-TC is shared by co-öp players. One person's gain may be used by the next. Co-öp also has some benefit in that as a player gains a restoration of his Aces or Seabag (armor and weapon) that can be handed to the other guy. Solo all Ace gains and restoration of Seabag are used by you the solo player.
Co-öp may be easy for the group to win, expect not all may survive. I died twice in Waterloo video, as my team won. I survived WW2 Zombies video but two others of my group did not.
You can also solo by entering with two or three characters all recorded and rotating between them the combat turns. Co-öp without the living (I guess). I sometimes do this in Alpha to test. You should see the deaths in test before I balance some aspects. I laugh knowing how the players would scream. We all love our characters, else we would not play adventure games.
Dice and luck do play a part. As the designer I do know how to use the rulez. Is a best practices needed? Don't think it's a frustrating learning curve, but you may need to lose a first game to see things as they are.
I also admit I make mistakes playing, in that, I will forget to add something or subtract something. Keeping track of the Fury Bonus is good for staving off age-induced memory decline. I tend to hold fingers out in front of me. You may also hear on the video how I am constantly repeating the number of the dice roll to keep focus on the task I'm resolving. I have fun but do cringe when I demonstrate and see how I cheated during video edit (replay).
Thanks for playing. I always find it flattering.