It's just memory, but the 40mb might run faster, I sorta felt that a little bit, but I could've just been out of it or something...
I have the 40mb one and it runs alot faster than my friends original one. Regardless, the space in the memory is alot more efficient, but that's it.
The 40GB can't play PS2 games and has 2 ubs slots instead of 4 and doesn't have a connection thing on the front. Also the originals (60GB) aren't sold anymore. and the memory is different hence the 40 and 60.