Though it's only a rumor according to the DP guide, according to my hands on research it's fact.
I've looked this up on the net, and SMS power says the US version is 1 megabit, where as the European version is 2 megabits. Today, I've confirmed this is true by testing both the European and US versions on my system at home.
Though I can't yet confirm any extra levels in the eurpean version, I CAN confirm some big differences.
In the first level alone, here are the differences I've found:
the Piper is easier to beat in the US version
The Sewer Rats aren't present in the US version
The Bats don't shoot at you in the US version
the Pumpkins that fall from the trees fall differently (easier) in the US version
The mid level Witch isn't in the US version
The End of level pumpkin dropping Witch isnt in the US version.
If you're interested in obtaining the 2-mega version of Captain Silver, the easiest way to tell the difference is that it has different (wimpy) box art, and a multilingual description on the back of the box.