Planet Earth SE 1080i

The UK Special Edition 1080i version will most likely be the best available if you have a player/TV that can play it.

PE was 'mastered' by the BBC as 25p for broadcast in 50i in the UK. In this way 'unclemat' on Blu-ray forum is right to say (in reply to your post there) 'think about it as 1080p25'. But he isn't entirely correct in saying 'the material was shot in 1080p25' as other frame-rates were also used, including high-speed and early cameras were likely 720p.

So the 50i version is the most original version, not the 24p slowed down version to cater for the US. Provided your playback chain can do decent de-interlacing and not converting to 60i it's not going to make a difference to the 'smoothness' you are concerned about.

There were long threads about the banding and flashing in some scenes in the 24p version but I never saw those on my Pioneer plasma.

As to which is more accurate in colour and gamma (ignoring the obviously faulty initial batch of the SE version), unless someone has seen the original master at the BBC, no one else can confidently say with authority.

50 fields per sec, 25 frames per sec, yes, matching the original master for that aspect of it anyway.

I wouldn't say that. It would be interesting to see screen caps of the same frames on all the different versions - though they won't tell you what it is like in motion. The US SE version at 60i will be different from the UK 50i SE version (which is the same rate as it's original master). Also, like Kilian.ca says, a lot of it will have been recorded at 720p (though someone also mentioned 1440x1080 cameras I think?).