One school of thought is that you should not invest too heavily in the company in which you work, because it creates a diversification problem. One of your major investments is your human capital (i.e., your job). If you also invest in the stock of the company for which you work, you're putting a lot of eggs in that basket. If you want to know what kind of problem it sets up, ask the folks who worked for, and invested in, Enron.
If you have a limited amount to invest, and you have to make a decision, add it to your 401K, and make sure that you 401K is stocked (a pun!) with a total stock market index fund + bond fund. Vanguard has some nice target retirement funds, if you have access to them.
Good luck, and I hope that's helpful!