If you know you want to be a PT, I would not recommend a PTA certificate or degree and then hope to move into PT school. I suggest you look closely at both careers and then determine what direction is what you want. Do not neglect to investigate the job market as well for each. PT's are and will be in high demand, especially as the population ages. PTA's may have a more difficult time finding employment.
So do some research on schooling required and job market before you decide. Good luck!