All the extra amplifier power provides, is the extra headroom. It has no impact on sound quality, or the frequency response of the device.
Whether it is worth it or not depends on how hard you are running the device to get the necessary SPL levels in-room. In a small room, with lots of room gain the user may not ever see any benefit from the extra power. In a large open room design, where the user pushed the device hard, you could conceivably benefit from the extra headroom. I don't have a good answer for which fits your case better.
If you are tight on money, the 500W is a good solution. If you have a pocket full of cash, the 1000W gives you more output. Whether or not you need it, is going to change room to room and individual to individual.
Kevin Haskins
Exodus Audio