I'm about 10 hours in and loving the game so far. However, on many occasions it is splitting the path I go down and making me choose what seem like major story points. I want to do both so here is a question for the GB players!

Is it worth grinding out the respect points and upgrading all my weapons and abilities? If I go through and complete the game will all of the cool stuff I've collected come with me to a second playthrough?

There's no New Game+ option I'm afraid. Not that I could find at least

I think what you could do is try to find a newbie co-op partner? Then at least you'd be able to take your decked-out character with you through another playthrough. I feel your pain, though. I hit level 50 respect as I finished the game, but I had already completed all the side missions. It was kinda cool to have invincibility and infinite ammo as I cleaned up some achievements, but I wish there was more.

If you're a pc player, there is a mod that lets you play through any of the previous missions again.

The splits have very little impact at all. The only one that does is the last one and the game just straight up let's you play through the other ending after finishing it so there is no need to worry about that stuff. Also you could just make a hard save before each decision.

I felt the same way and I am hoping if my roommate ever decides to play games again that I can convince him to do the choices I did not pick. I am glad I did not wait for him though I have had it almost a month now and he still hasn't touched the game ><

You can do the end mission both ways and I would say the only two missions that are worth replaying for the splits are Murderbowl (of whatever that one is called) and - to a lesser extent - the zombie one. so do a separate hard save before each of these and go back later to see the variants.