Buying into Quasi-Monopoly’s

Hey, I have a question regarding quasi-monopoly’s (or oligopoly‘s) of a specific sector.

For example: S&P Global and Moodys (that way you have all publicly traded credit rating institutions)

My question would be:
would you rather buy both companies and thus buy the „whole cake“ or would you rather just buy your preferred company (of course if financials look ok etc)?

I think the risk of losing would be much less if you just buy the whole cake. But I’m kinda on the fence right now.

I hope that you get my point 🙂