well there are 6 editions to vista. even though 2 of them dont appear in the shops.Plus the enterprise edition.
Home Basic
Home Basic N
Home Premium
Business
Business N
Ultimate

It Depends on what you want to use them for.

http://www.microsoft.com/uk/windows/products/windowsvista/editions/n/choose.mspx

that website will show you them all compared and what they do and what features etc.

Everyone seems to be getting ultimate version ,(like i have) because its the best one. If you buy the OEM edition around