I don't think anyone should give you a solid recommendation without knowing more about the tank itself and the environment it will be located in. Here's some things to keep in mind that will help influence your decision. IMO everyone means well with the recommendations so far but for the most part it's just a mixture of names people have come to trust. Heating/cooling will be different from one 500 gallon tank to another. When you start getting into these sizes it becomes more important to consider the science behind it all.
- What is the insulating value of the tank itself? Acrylic is often used for larger tanks and will insulate your tank roughly 5 times better than glass would.
- What is the ambient room temperature on all sides of the tank?
- Are and sides of the tank insulated (commonly there's additional R value on the bottom, sometimes back)?
- Is there air conditioning in use?
- Is there any heat loss from an open top or plumbing taking water elsewhere (eg. sump, basement sump)?
- When you do a water change are you pre-heating the water, if not how much are you changing and what temp?
- If you're planning to place the heaters in a sump what sort of turnover are you expecting? (a slower moving sump may not be able to adequately keep up with excessive heat loss from cold climates)
- What duty cycle can the prospective heater support? Keep in mind not everything can run continuously and similarly frequently cycling anything can cause premature failure.