Lines is a TStrings, a class that is basically a wrapper for an array of strings.
Text is a simple property that when read concatenates the strings together (with lineseparators between them), and when assigned too parses the single string into multiple strings.
I wouldn't worry too much about it, just think twice before using it for huge strings (think hundreds of MBs and bigger), since all the copying done by this highlevel functionality will eat quite some memory.
Another (minor) reason not to use it is if you want reading and writing to be binary the same. Assume you have a text with mixed line endings and you assign it to text and read it back, then the mixed lineendings will be uniform now.