Arts like Taijiquan have Fa Jing and Hua Jing but arts like Xingyiquan have Fa Jin and Hua Jin. What's going on here?

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).

Jin means strength and is a refinement of a basic form of internal strength that comes from Standing Meditation. In Xingyiquan there are three levels of practice; Ming Jin 明勁, An Jin 喑勁, Hua Jin 化勁.

The idea of Xingyi's three levels of practice came from Guo Yun-Shen (郭雲深) and was systematized by Sun Lu-Tang (孫祿堂) who proposed the levels as follows:

1. training the Jing to transform into the Qi 一練精化氣,
2. training the Qi to transform into Shen 二練氣化神 and
3. training the Shen to return to emptiness三練神還虛.

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.