@Xenocrates: That's EXACTLY what I'm wondering. I have no doubt that National Park outweighs any other nat. wonders in your GP farm (with the obvious exception of nat. epic)...but is it really that good that it outweighs the disadvantage of getting it that much later? Over the course of a whole game, which method would produce the greatest TOTAL number of Great people? Surely that's the most effective. If the food method just pips the forest method in total, even though the forest method is better at the end, then surely the food method is the best.
I think this question can only be solved by working out the absolute maximum no. of specialists a food-based GP farm can support, and how many a forest/national park city can support, and guesstimating the total number of GP points produced throughout the course of an entire game...
I think it really depeds on the lay of the land and what your stategy is more than anything. Saving forests around your capital is only going to work in very specific situations where you a) have the forests, b) have the time to not chop them, c) have enough food resources to live without what beneath the forest canpoy. So that is super rare, and might be impossible at higher levels I would guess.
What I find a lot is my closest neighbor has a high forest capital, and not being as aggresive about getting Bronze Working as I am, when I capture it with an early rush, all the food resources are worked, but very few trees are chopped. That makes an ideal candidate, since you don't have to worry as much about growth at that point, the city will be an OK producer with the forests, and, if the food resources are good enough, you can use specialists there and its an OK early GP farm as well. In this case, I would guess this kind of city makes the most GP, since early it can be a specialist based GP farm (and might have a wonder or two as well), and late its an ubar NP/NE mega-eco-city. In this case, I would guess your GP production is maximized. But I'm still a bit of a newb, I just happen to have run into this a bunch. I've read people believe two GP farms is optimal now.
But, if you get a really productive Capital and can wonderspam, making NE in it right away might be better in the long run. Or if you find a really ideal GP farm candidate, or if you just don't have any good options for a NP/NE farm. I bet the majority of games are like this.
What I did notice with the couple of really ubar NP/NE cities I've had is the initial GP rate is insanely high. In the middle stages I'm trying to grow the population of that city to 20, in order to cover all the forest preserves, so I'm not using specialists there and my GP rate goes down. Once I hit NP, my rate goes way up to the 200 GPP range suddenly and a bunch of GP pop out very quickly. I timed it with a Golden Age once and it was just amazing. This can make a really awesome timing synergy with Cultural Victories. Because most of the game I am not worrying too much about culture, just preparing with spreading religions, temples, cathedrals, specific wonders, making a strong defensive military, etc. My focus is really teching to Radio which is where I like to flip the switch and do 100% culture, and all specialists focus on artist. If you time it right with the NP coming online you immediately get a large number of great artists to help your push to 3 legendary cities. Can you imagine all the Hippy Poets and Artists hanging out in the Forest Preserves making great works of art and dancing around naked?