Suspension or rigid, it depends on what you are using it for. If you stay away from rough ground, then you have no use for suspension. If you ride over rocks, ruts and roots, suspension is great. I switched to full suspension 9 years ago and I will never ride a hardtail again. With full suspension, you can pick any line you want, or no line. The brakes work better, everything is better.
What everyone has said above is true, though, about the money part. You're better off with a decent hardtail instead of a cheapo suspended bike if you only have $500. I would suggest used might be an option. Again, decide what you want out of it.