All things being equal, if the same IPTV service (I don't know who offers both) was loaded on the same box with the same internet connection via both M3u and MAC address, would one method of connection tend to be better (eg. more reliable, less buffering, less lag in changing stations) than the other?
M3U and MAC is simply a connection,
MAC access simply you connect to server, use a username and password and it matches MAC and allows access, if MAC changes it wont allow unless you update.
M3U its based on URL and containes username and pass. Allows you to change devices and even use on mobile providing link is not being used.
Some MAC based providers are better as they have own interface but some M3U are better, there is no right or worng. Its more whats better for you and what you like. MAC some will provide own interface or STB emu, M3U u have more flexability