Just for explanation, here are some meta posts that clarify the position of Chem/Phys SE on using Mathjax in titles.
Chem: https://chemistry.meta.stackexchange.com/questions/261/is-mathjax-in-titles-a-problem-and-why, https://chemistry.meta.stackexchange.com/questions/253/should-we-revisit-the-url-slug-issue/254#254
To summarize, Chem takes a harder stance against Mathjax and says if there is any reasonable alternative, it should used. They feel the issues it causes with rendering on other sites and searching for questions aren't worth the improved formatting.
Phys sets a lower bar for what questions "need" Mathjax in the titles and feels that Unicode can lead to an inconsistent appearance of questions on the site.
It seems to me to come down to whether they want to emphasize internal or external use of the site. Mathjax looks nicer for existing users, but may cause problems for searches down the line. Unicode isn't as well formatted, but external searches will have an easier time finding these questions.