I don't think legitimacy is a meaningful concept in this context. I would use the chain with the greatest utility, which is determined by whether I like the rules, whether the rules are consistently and predictably enforced, and whether other people I want to interact with are using that chain. Whether the people who use the chain have more Ether or more computer power compared to the people who use a competing chain doesn't affect utility.

While I don't think it's meaningful to say that one chain is more legitimate than another, if I was forced to make such a deamination in this case, I would consider the chain where the rules are consistently and predictably enforced in a way that leads to predictable outcomes more legitimate than a chain where validators have colluded to arbitrarily subvert the enforcement of the rules.