I prefer working out by myself. I bull-shit less and get more work done in less time.
The only thing that takes a little more time is setting up the pins in the power rack so I don't need a spotter, but that's easy enough. Works for any exercise.
I never understood why a gym would have 10 benches but only one power rack.
On the other hand, having a training partner could help with staying motivated. I'm more likely not to skip a workout if I know someone's counting on me to be there.
It all depends on the partner. My last training partner never understood why I would pay attention to my rests between sets and would even start asking me non-workout related questions while I'm in the middle of a set, and he'd get pissed if I 'ignore' him because I didn't answer right away. He still uses whatever brand new supplement comes out without researching it, and has never squatted or deadlifted (any variation) in his life.