First of all, you are in the enviable position of having two attractive options to choose between. Whether you choose to do the PhD, or continue in the start-up company, you have bright opportunities.

I don't know anything about your field of study, but I have been in a similar situation to yours. I had a difficult choice between taking a University degree or taking an offer for an interesting job straight after A-levels. I very much wanted to go to Uni, but realised that the job opportunity was probably a once-in-a-lifetime chance. The degree course would always be there, if the job didn't work out.

Perhaps taking a similar thought process might help with your decision. If you stay working at the company, would there still be PhD opportunities for you in a few years time? If you do the PhD now, are you likely to find another start-up company with the same prospects as this one?