It's a tough choice but I voted for Siam. You get more benefits out of CS', if you have a rapidly spreading religion with papal primacy + double CS religious pressure enhancer + patronage, you effectively replicate Greece's UA and can maintain friends with every single CS. Eventually, more food, faith, culture beats less gold spent on maintaining allies. Then again, with Greece, how low can you reduce CS influence? You got the UA (-50% decay), patronage opener (-25% decay) and religion (-25% decay). I know that patronage opener and religion in CS stack (so -1 goes to 0.75 and then to 0.5). I guess the lowest influence decay is 0.25 for Greece. The main counter for Greece is spies though. Piety for charitable missions is also ideal for both of these civs but mostly Siam.