Owner of Web Deisgn outfit Armadillo Studios, Creator of the yycApps and AlbertaTweets projects, iPhone App investor. I love to give advice on starting up your own design agency or iPhone app company. I also hate Helevetica.
If money and security is the sole object, then it may more sense to take ad agency gig. But if freedom and a bigger stake in the overall direction and of company is a priority then staying the course as a freelancer may make more sense.
To me there are other factors that would come into play more than money. I imagine a Freelancer growing a business (300K) is probably spending much of their free time focused on a business and not much free time to enjoy life. There is also the instability of running a growing business which would play into it as a factor. Working for an Ad Agency might reduce those stressed, but at a 100K salary I imagine there will be other work/life balances and requests.
I don't think there is an easy answer and it all depends on the personal and professional goals of the individual. I know that's not a concrete answer, but hopefully it does help.
I would suggest looking within your local Tech community. Search out various tech centric events through Meetup.com or look for something like a DemoCamp (or a Camp like event). Those events will attract people within the tech community that are interested in either new ideas, concepts or potentially becoming a cofounder. You could also look to see if your city has a Start-up Weekend. If you're comfortable, presenting your concept or through an event like that to see if you can match up with a passionate tech cofounder.