On the one hand, great people rock, especially in the early game.
On the other hand, having solid alliances with city states also rocks, especially in the early game. Lucked into 100+ influence with 3 city states (maritime, mercantile, and cultural) through a combination of quests and barbarian kills in a recent game, and that propelled my growth and expansion far more than any three great people could have.
Another thing to take note of is that, coup attempts aside, G&K is set up in such a way that once you get an ultra-strong alliance with a city state you can often maintain it without really having to do anything. Towards the middle- & endgame they're constantly throwing out 30-turn challenges to produce the most faith, culture, or discover the most technologies...if you're beating the AI in those areas then that, along with Patriarchy & a shared religion, will keep you allies on its own. Throw in a few wonders built here and there and next thing you know you've got 300+ influence.
So, yeah...if I had the choice between planting a great scientist or grabbing a solid & long-lasting alliance with a good city state in the early game, I'm taking the city state.