The curved one is called a Panda cabinet and is a Game King/Igame +. The older one is an original Game King. As you noted it doesn't have the stereo speakers. It also doesn't have any provisions for a second row of button switches. The motherboard in the older one is different than the newer machines, as is the power supply, and I/O card. It probably has an older DBV-200 bill validator in it and may have a 3802 MPU board unless someone has upgraded it. Parts for these older machines can be somewhat difficult to find, particularly the power supplies. With some modifications you can adapt it to work with newer ones, but that may involve repining the power connector and creating some type of mounting brackets for it.
Is it worth $500? Depends on the market where you live. That would be on the high end of the scale for me being an older machine. The condition of the machine (working?) and the CRT would be the biggest concerns for me. If the chrome is worn/scratched to hell and the CRT is has bad image burn I'd definitely let it pass. Those older 19" cabinets are large (and heavy) too. They're pretty deep due to the larger (and longer) CRT. Keep that in mind when thinking about where you'd put it.
Good Luck!