In a position to save

Hello,

I am considering options of investing with a 401k and a stock purchase plan and can invest around 20% of each paycheck into a combination of either. My company has a 401k match up to 6% starting in 2017.

I am 29 and I feel 401k would yield more due to compound interest. (The stock is only 10% discounted from the purchase price and freezes moving the money for a year). Is it better to invest in 401k at the age of 29 due to years of compound interest accumulation? My company is fairly matured and I don't foresee the stock doubling or tripling in the next 4-5 years. That being said the stock has doubled in value in the past 5 years.