While both string and text properties are used to store string content, there are 2 major differences between them which you should consider when choosing one over the other.
1. Storage differences
String properties are intended to store small to medium strings with a length less than 400 characters.
Text properties on the other hand can contain arbitrarily large strings (typically blog content, descriptions etc).
Depending on the size of your content you can decide which property you would like to use.
2. Search differences
All properties inside Appacitive are indexed for searching. Given their size and general usage string properties are indexed for both full text as well as equality searches. However, text properties are only full text indexed and you will not be able to use the equality (==) operators when searching on these properties.
Text properties also cannot be used when sorting results for a specific type.