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.
I played co-op for the first time last night/this morning! It was fun! We did three random missions for the trophy and then just messed around fishing and doing whatever. It can be easier but if your play styles are different you have to adjust! I like to sneak in...her? Not so much! LOL!