I don't know all the details, but I believe that the Unlimited version has all the planes unlocked right from the beginning, no codes needed...not sure what else it has different, though.
What the above post says, and..
the score counter doesn't go all weird (like in Garegga) when you get to 10 million points on the unlimited ver, and the score counter doesn't stop at 63m, like the original version does.