You can certainly try to escalate the matter with Advantage corporate to have them pay the difference, but my guess is that it will be an uphill battle. Supposedly a rental reservation is a binding contract, and if the rental company can't deliver on what was promised then they are obligated to make good on that, which in this case means compensating you for renting the vehicle you reserved elsewhere.

While I don't know if anyone has ever gone this route, you could sue them in small claims court for the difference in cost. They might just reimburse you to avoid the hassle of showing up in court, but it's up to you whether you feel it's worth your time and energy.

I will say though, that many reservation systems do not have 100% accurate real-time availability, so it's very likely that the rep at Advantage thought the other company he was sending you to had a convertible available, even though they were actually sold out.

As you found, there can be pitfalls renting from some of the deep-discount brands vs. the majors. Sometimes you don't actually save money in the end.