Congratulations on booking your cruise! You'll find a lot of things have changed in the world of cruising since 1990.
Early or late seating: what time do you usually eat dinner? Advantages of early seat include finishing with tons of time to spare to enjoy the ship for several hours after dinner. And if you usually eat dinner any time around 6 or 6:30pm, this is your seating.
Advantages to late seating include time to shower, change, relax, have a drink or do some dancing before dinner. No rushing home from shore. Being able to have an afternoon snack or "tea" and be hungry for dinner. Disadvantages include getting out of dinner at 10:30pm and not having a lot of time to hit the lounges, casino, etc.
I'm sure other folks will add advantages and disadvantagese to both seatings as well.