Is it better to have a single-author paper or a joint-authored paper?
You should really listen to what your advisor says. "...he has been pushing us to combine our results into one paper." That about says it. Your advisor knows what works and what doesn't work as a paper in your field. He also knows (presumably) about the scope of your work and how it fits into what you and the other students have been doing. 

There is little downside to this. The importance of the paper to your career has far more to do with how well received the paper is than with how many authors there are. Having more authors means the paper is stronger, means you have a network of people to help rewrite and deal with any negative reviews you may get, and (as suggested in Peter's answers) it shows you can collaborate with others. 

It would not hurt to bring this up with your advisor, make your point (ask your question), and see how the advisor reacts.