The optimum system would involve both methods. In the labs I have worked in you take the frozen stock and streak it onto a slant or plate. From there you innoculate media the following day.
Some reasons why growth is faster from plates vs frozen stocks are lag time from that deep sleep, the lack of complex nutrients agar-wort media can provide and the fact that over time cells in a glycerine suspension will die. That is why it is a good idea to streak before innoculating, to obtain a pure viable cell line that is proven to grow.
Both methods are relatively cheap. I currently have both but have opted for buying fresh yeast because of the variety and ease. I was lucky enough to find old petri dishes on ebay that came with a metal container.
In any case if you ever decide you want to deep-freeze a strain because you won't brew with it for around a year or you prefer one of those limited edition releases that aren't always available it's handy to have glycerin. Plus a backup stock is always useful when you get some random contamination.
I always keep a plate of Bock yeast in the fridge ready to go, but I will probably freeze the limited edition english ale yeast I am pitching this weekend if it turns out good.