B+ Trees
The difference between a simple prefix B+ Tree and a plain B+ Tree is that the plain B+ Tree does not involve the use of prefixes as separators. Instead, the separators in the index set are simply copies of the actual keys.
Simple Prefix B+ Tree are often more desirable than plain B+ Trees because the prefix separators take up less space than the full keys.
B+ Trees, however, are sometimes more desirable since 1) they do not need variable length separator fields and 2) some key sets are not always easy to compress effectively.