I think of the Animatrix when I think of signing treaties with AI robots.
Zombies are indeed worse on a psycological level but on a survival level I'd say the Mechs have the upper hand.
Then again, situations change and a war of these sorts could play out many different ways. For example... Are we talking about "Night of the Living Dead" zombies or "Dawn of the Dead" (2004) Zombies? For that matter, what about 28 Weeks later zombies?
Survival rates on Original Romero Zombies- High, slow walking head shot targets.
Dawn of the Dead 2004 Zombies- Low, fast and still undead.
28 Weeks Later Zombies- Medium, fast but not undead (multiple body shots can kill) and you can win out with time (as they die not too long after infected).
Mobile Suits controlled by AI... Hard to say...
We talking about robot civilizations from the Animatrix or something along the lines of "Christine" crossed with Gundam?
With a Christine MS I'd say we could take it down no problem.
With an Animatrix civilization producing AI controlled Gundams like Corollas...
I suppose any of those could be a good story somehow.