I think that showing the buttons, but disabled, is a better indicatorFor cases like this, where the inability is circumstantial, I concur with J that disabled (and desaturated, or "greyed out") is preferable. Especially in a case like this where the reason it is disabled is manifest on the page: you can see that there's nothing to compare with.
(I wonder if that's a good design rule: ensure that the reason why a control is disabled is visible. Either implicitly, as with the Compare button, or explicitly when needed.)
The opposite case are things like the Admin pages not being present at all for a Member (as opposed to mod or owner). The change of Role is a major change in the semantics of the site, and for most users something that never happens. Leaving something completely out of their control visible but disabled would be pointless and would generate needless questions.
In between are things controlled by moderator permissions. I think these are more like member/mod distinctions (except possibly for mods with the permission to Set Moderator Permissions). If those are currently hidden (I haven't checked) I'd probably leave them that way.