Yes, Cracker, we had a long debate about this in the HoF thread. Metropolises do offer a higher potential (because for every extra city center you have, ICS gets a content person out of it, instead of a happy person). ICS gets a high population faster, but the more spaced out approach has more long-term potential. The real question is whether or not the more spaced out approach has enough time to catch up to make up for the jump-start ICS gets.
Yes, you can join workers into the cities, but you also have to build the hospitals (and mass transits-building these in 100's of cities can take quite a while), and have enough happy improvements to keep everyone happy (which is hard to do in cities where luxury spending does no good).
If you build on grassland, you do get only food for 1 person, instead of 2. Plains, you would lose out on 1 food (half a person). Building directly on desert tiles, hills, etc. you get more food, but a content person instead of a happy person using that tile. Overall, this comes out to about a 10% difference in the internal game score, but if you can achieve the max population more than 10% sooner, you come out ahead in score.
I think for ICS, it will pay to finish somewhat early for the Jason score, and for the more spaced out approach, finish later. This depends, though, greatly on the land available (enough land for the settler flood to work). GOTM17, I shouldn't have built so dense, as land was hard to get, and it showed by EMan beating me by 2-300 pts.