רב חיים הלוי says there is a doubt about this, and רב שך brings a proof that the רמב''ם does hold with the רשב''ם that the גנב can pay שווה כסף
The proof I have that the רמב''ם does not hold by the רשב''ם is from הלכות גניבה פרק א' הלכה י''ד.
My reason is simple. If the רמב''ם would hold by the רשב''ם then why does he not write simply if the גנב broke the כלי we evaluate it according to the time of העמדה בדין? Why does he divide הלכה י''ד into two parts? One is which the value of the כלי went down and we go by שעת הגניבה and part two is where it went up in value and then he broke it and we go by שעת העמדה בדין?
Part one by itself is not a question on the idea that that רמב''ם hold from the רשב''ם because it only is referring to a case where the כלי was not broken. But if we look at הלכה י''ד in its entirely it is obvious something is missing in part one. That is the case where the כלי went down in value and then it was broken. If the רמב''ם really would be holding from the רשב''ם then he would say if the vessel went down or up in value and then it was broken we go by שעת העמדה בדין.
So instead according to the way I see it, אין שמין has nothing to do with the time of evaluation but the fact that the thief must give back כלים שלמים or כסף.
Right before I broke my leg, I was thinking that Rav Shach's proof that the Rambam hold like the Rashbam is not a strong proof. I do not know if I wrote my thoughts down anywhere but as far as I recall Rav Shach was building on the idea that the owner of the broken vessel can ask for the pieces back. If the Rambam would hold like Rashi how could that make sense? But the way I see it even if the Rambam hold like the Rashbam this still makes is a problem. If after all the thief owns the broken object, what gives the owner the right to ask for the pieces back? So you have to say this is just a special thing to allow the owner to ask for the pieces back.
I might just mention the important fact that the Rambam does say the thief pays back דמים money. If he would be holding like the Rashbam that at least seems curious.