This is a very contentious topic on some other boards. However, given the way you ask the question, you should probably choose As You Wish.
Up until a few years ago, practically all Main Dining Room service was based on First and Second Seating. You would sit at the same table, same table mates, same wait staff and same time every night. Some people loved this, others like me, considered it rigid, inflexible, restrictive and silly.
With the addition of multiple dining venues and upcharge locations, where you pay additional beyond the cruise fare, the lines realized they had to make flexible options in every venue, including the Main Dining Rooms. Royal Caribbean calls it My Time Dining while Norwegian has abandoned Seatings altogether.
Cruising on Norwegian and Royal, I've never had a problem getting a table in a Dining Room as a walk-up. You can make reservations, but it's not really required.