Should the mean energy be written in angular brackets or with an overline?
Both the IUPAC 2008 Green Book (section 4.2) as well as the ACS Style Guide (specifically Chapter 11) agree that both methods of representing the mean value are acceptable. There might be some style guides / international standards that state otherwise, but for the average chemist these two are generally sufficient.

If ever in doubt, just state the meaning in text.