What is likely to bring you a greater return is the blue chip stock, for one thing because over a span of years blue chip stocks average 8-11% return (depending on which span of years). Another factor in favor of the stock is the fact that it is a lot more risky than a bank savings account (which is insured). Greater risk is typically rewarded with greater returns overall. A 15-year period is enough to justify the greater risk, because most business and investment cycles span fewer years. Which company you choose for your blue chip stock can make a huge difference, of course. Some "blue chips" of five years ago are bankrupt now!
Another consideration is the specific percentage return. 2.5% interest on a savings account right now would be phenomenal! Back in the '80s, though, when banks were paying "only" 6% or so, investors were opting for money market accounts, which might pay 10 or 12% or better! The current economic crisis has brought interest rates to historical lows, so it would be unwise to commit long-term money to even what would be considered a high 2 1/2% rate now. In five years, rates could be double what they are now. And when rates do rise, it's likely to slow corporate growth.
Overall, my faith would be in the blue chip stock (carefuly selected) over the guaranteed interest on a certificate of deposit through a bank.