Smallpox vs. largepox, gen2 vs gen5, early war vs. peaceful alliances. Which is your favourite gaming style?
OTOH, there's also the "Cowboys and Indians" strategy: Republic in the early game to massively increase technology and have better units, ships and Factories for the bloodbath later on when you switch to Monarchy or Communism. It also gives you lots of tax to spend on rush buys and diplomatic actions, such as Inciting Revolts in cities too close to your land. That technology can also allow you to build all the wonders before the other civilizations have researched them.