I prefer going solo, that way I can go at my own pace and don't feel like you have to do missions, side missions, outposts or prepper stashes to keep things entertaining for you personally. Hell, I cant count how many times I've been heading to a mission and just fucked off to just go hunting or fishing for a good hour or so at a time. But I will say that it is helpful to have someone else playing with you because they can revive you in a battle. My co-op partner helped me out quite a bit with that when I took on Faith. And the co-op trophy is pretty damn easy too, just do three prepper stashes you can get it within 30 minutes.