Text Clips and Code Templates are very similar in the effect they have on text (the code in PN is identical for both), and are differentiated by the way you access, use and edit them. Here is a quick comparison:
Code Templates allow you to type out new code very quickly using short keywords - they're probably best for frequently used fragments. Text clips are more useful for browsing big libraries of clips, perhaps those you access less often.
There probably will be UI for editing text clips in PN soon.