Jing means refined, proficiency, skill, or precise. It is a term which describes the result of a practice. So Peng Jing defines a sort of energy or power or feeling you get which is the result of a basic exercise (jibengong).
A short look at this list seems to imply that by Guo Yun-Shen and Sun Lu-Tang's analysis, there is not a lot of difference between the concept of jing and jin. The actual energies may be different (they are different arts after all) but the concept and terms used to define them are nearly interchangeable. However for consistency it seems to be fair to write jin when talking about Xingyiquan and jing when talking about Taijiquan.