I think one representative per species/race is a better idea.
I think that's a terrible idea. If a Tellarite is born on a mostly-Human Mars, he's raised within Martian culture, his economic interests are in the success of the Martian economy, his family is on Mars, etc. -- why should he have to vote for the "Federation Councillor for Tellarites," the majority of whose constituents live on an entirely different planet and may have entirely different, or even conflicting, cultural and economic interests? And those Humans who live on Mars may have totally different cultural and economic interests from Humans who live on Earth. Etc.
If it was one vote per world all folks have to do is spread out their populations on different planets and have a huge voting block.
So I think we gotta be more specific in our terminology.
It's all fine and dandy to equate "world" or "planet" with "Federation Member" when speaking informally. But if we're getting formal, I say we think about it this way:
The United States don't have the State of Hawai'i Island, the State of Maui, the State of O'ahu, the State of Kaua'i, the State of Moloka'i, the State of Lānaʻi, the State of Niʻhau, and the State of Kaho'olawe. Instead, even though they are separate islands, they are all part of
one state -- the State of Hawaii. There isn't a separate State of Long Island -- it's part of the State of New York. Etc. Because membership in the federal union is not about geography per se -- it's about
polity. About political organization.
So really, we shouldn't be thinking in terms of individual planets, since multiple planets may themselves be part of a single polity. It would be kind of silly for Luna to be its own separate Federation Member just because it happens to be a separate celestial body from Earth.
We should be thinking in terms of Federation Member States rather than planets, just like we think in terms of the State of Hawaii rather than individual islands.
There
is a valid question about when the inhabitants of one planet or another ought to be considered as a distinct polity from whatever polity they previously belonged to. But I think that points to the question of, what ought to qualify a polity for Federation Membership?