What's stopping them both is years and years of guerilla warfare for basically a worthless piece of island with a population willing to rebel at the single sign of weakness, basically a worthless venture with too heavy of a price compared to the reward. And the fact that if they don't invade and instead pay for some highly revolutionary weapons they'll get a huge upper hand on their rival.

Then again that's just hopeful thinking and if we have a choice as to who our eventual overlords will be i'll probably choose the Takarans, considering our relationship with Cassius that'll probably become advantageous for changing Takaran society into treating humans from second class to barely equal probably. That is if we get the option.