$300k isn't a life changing amount obviously (in the lavish multi-millionaire sense), yet if you have debt it could pay off perhaps (hopefully?) all the debt you have and put a big down payment on a house depending on where you're at, etc.
Point being, that decision is very personal in nature, it depends on your circumstances. So you want to start there, with your personal circumstances. Then after that, it depends on what you think of the company prospects. If you think the company is going to be far more valuable in ten years, regardless of the present stock market bubble, then that should influence your choice. I would suggest avoiding trying to guess where the stock market bubble is going to go, who knows if it's going even higher yet or if it's just about to pop; and focus on things you can control and understand with certainty.
Finally, obviously you don't have to liquidate all of your holdings. If you have $50k in student loans, sell at least enough to wipe that out. Debt is one of the few things I overwhelmingly will always argue in favor of eliminating at the first opportunity. There are a lot of ways to earn $300,000. Debt can and often will make your life a living hell on the other hand.