johnny b,
If you decide to enroll in the Teamster 401(k) and later decide to accept a management position (go company?), you should be able to roll over the balance into the company-sponsored 401(k) without any difficulties. Seperately, whatever Teamster pension credits you have earned however, are not portable. If you buy shares, you will have to pay taxes on dividends and if you decide to sell, possible capital gains taxes. Simply put, the 401(k) is a pre-tax plan, while purchasing shares is an after-tax plan.
Don't take this post as gospel, read the 401(k) and stock purchase plan prospectuses yourself to determine what is best for you. If you enroll in both, you may want to consider how to diversify your holdings. UPS stock is a component of the S&P 500, for example.