I like co-op the best. Don't have to worry about all the balancing and bitching because the AI won't complain when it gets beat. Plus it allows for actual diverse gear and set ups. Most competitive games have a meta you have to abide by or you'll just get beat by people who are using it. That's not to say competition isn't fun because it is, sometimes you just need to test yourself against other real players, but if I had to pick one id go co-op.