I've done both and my preference depends on the situation. For example, I spent eight months doing one lap around Australia and did that tour solo. I also spent six months cycling from Amsterdam to Vladivostok and did that trip with another cyclist.

Cycling alone around Australia worked well since it was English speaking country and pretty easy to tour. The trip across Russia also worked well, since having only limited Russian speaking skills it was nice to have deeper conversations with someone. Also two of us were able to problem solve through some issues. What we did figure out pretty quickly were that our cycling speeds were mismatched, so we developed a rhythm that worked with me taking off earlier in morning - meeting up closer to lunch and then sync up at end of day before finding a place to camp. Except for a 10 day period after we missed each other on the road and thus traveled independently, worked well to cycle with two of us.

I've also done mixes close to one another:
- First was the Russia trip with one other person
- Followed immediately by two months of a TDA ride across China with ~50 people
- Followed by a month of solo cycling through Thailand
Each of those phases was fun in own way. All other things being equal - if I am fluent in local language, I tend to prefer solo - and where there are additional challenges (e.g. Russia, Africa, China) it is also nice to work through those with others.