Well lot of people wonders when you should use a sub domain and when a sub folder since google have pretty much made them same. So I just finished writing the Post on my blog on When to use a sub domain and when subfolder. I have put 4 points which you should look at to decide whether you need a subdomain or not. To summarize the whole post, I think if the newly added content fits into current theme, you may like to go with subfolder otherwise going with new sub domain may be a good idea.