There is enough of a difference between some versions so it is difficult to compare scores across them.
Which to choose is largely a matter of personal preference. The Japanese version plays much slower, with both pacman and the ghosts moving more slowly. This allows more time for reactions and decisions to take place and makes the gameplay a little easier. I seem to end up with more time at the end of the round on the Japanese version as well. The power pills don't seem to go as far though - I'm not sure if they have different timing or whether it just takes longer to catch the ghosts with the slower pacman. It does mean that you need to spend more time grouping the ghosts to get an L-Pac in the Japanese version.
Both the "world" and "midway" editions play much quicker, and have a different sprite for pac-man. There are some difference between these in terms of level layout ("world" is closer to the Japanese levels), but these are somewhat more comparable in terms of scores. I prefer these versions partly because the version I played as a kid was one of these (not sure which one - possibly the world ed). The gameplay is faster and more entertaining, plus the game ends up being a bit shorter (a million point game is over half an hour long if starting from trip 1). You need faster reactions and decision making, but it is easier to get ghosts more spread out.
I do find it difficult to switch between the Japanese and midway/world editions, mainly because the timing is a bit different. So it is probably easier to choose and stick with one. The Japanese version is more popular amongst Japanese players (unsurprisingly) and the world/midway ones more popular in America/Europe I find.
Twin galaxies don't appear to specify a version, which is a bit of an oversight I think, as the versions are not comparable, although most of the records are old ones from the US, so I suspect they used the midway version. The 4 million score by Mark Mendes is a little "out there", not sure if that will be beaten any time soon (or ever?).
MARP records are around 2 million for the Japanese version and just over 1 million for the midway/world edition. I would guess at the latter of those being easier to beat, but as I said above, difficult to compare.