Label and Text have different CSS properties. Label inherits from Labeled, Control, and Region, which means it inherits a great many "styleable" properties which Text doesn't have. A Label can have alignment, a graphic, a background, a border, a displayed keyboard mnemonic, built-in wrapping, and can be intelligently clipped with an ellipsis ("…").
Text can participate in a TextFlow. (Technically, Label can also, but it is treated as just another embedded control and is not laid out as text.)
In general, if you want to show the purpose of an input control by placing one or more words next to it, and/or you want to allow direct keyboard navigation to an input control, you use a Label. If you want to display text content not associated with input, you use Text.