Snickersnack: The steam version has steam cloud support and achievements.
Hm, I can't claim I really have a clue what those features entail but from what I've read I think I wouldn't miss the cloud support and as far as achievements go, I'm not sure yet if I consider them advantages or disadvantages. I'm not that interested in comparing high scores and I'm afraid that achievements might feel like a Director's Cut to me, something that might ruin the experience of a purist who just wanted to play the original game the way it was meant to be ... (Yeah, I'm prejudiced like that.) ;)
Any other differences?
BTW, are there likely to be any consequences if you fake the billing information when paying via PayPal? IMO they don't really need this personal data when someone makes an online purchase via PayPal, I suspect it's only required because they were too lazy to change the system they use for credit card payment, and I'm sick of spreading my personal data all over the internet (e.g. giving out a telephone number when all I want is to buy a game).
(Heh, sounds like I'm not really the type of customer Steam is looking for ... I just wish all DD sites were as uncomplicated and DRM-free as GOG. :( )