Hmmm, well to be honest they'd both work. So why not both? The Rimworlders would despise the idea of joining an organization like the Commonwealth (even without realizing the drawbacks of them not being citizens they'd hate for how controlling it is) and the Sauru'u, well, being invaded isn't exactly something anyone desires.

How old are the Sauru'u, though? Just to get out a rough timeline; since the conflicts between the Sauru'u and Commonwealth with the Rim don't necessarily need to be around the same time.