Different classes for the same fund is just a way that the company can offer a lower expense ratio to higher-balance investors.
I.e. the "Investor" fund may have a minimum balance of $2,500 for an ER of 0.20, whereas the "Advantage" has a minimum balance of $10,000 with an ER of 0.15, and the "Institutional" class may have a minimum balance of $1,000,000 for an ER of 0.05. However, the funds are exactly the same otherwise, and (I believe) the money is actually managed as one big pot.
Vanguard has operated this way for a while, but I've only noticed Fidelity doing it recently. If you have an investment in a lower "class" than you are eligible for, they will actually automatically upgrade your shares to the higher class.
They also offer higher classes for their larger 401(k) plans. So, while the Institutional shares normally have a minimum balance of $1,000,000, but if, say your company's 401(k) has a total balance of $10M, they may offer Institutional shares as part of the plan.
Bottom line, you should buy into the highest class for which you are eligible, because the only difference is expense ratio.