I also think the Gold/Silver version is more desirable and should fetch more than the solid silvers.
Incorrect.
+1. I am an avid coin collector and have every silver coin Casascius produced. The all silver ones are clearly more valuable/desirable.
But you don't have the 1K gold coin. Unless you were the one who bought it for $1M. I almost have every coin too. Multiples. Except the 1K gold one or any of the bars.
The solid-silvers lack sex appeal. There is a reason Mike displays the 10BTC Silver-Gold as his avatar and on the Casascius website.
I would pay more for the Silver-Gold than the Solid-Silver any day.
They are so sexy!