Logs burn for much longer than planks, but there are fewer of them.
For crafting, it’s most efficient to use loads of splintered wood than 1 log, because no matter how small the fire is (1 string), you can cook 20 cooked meats. Each individual splintered wood lasts way shorter than a log, but if you have 50 splintered woods vs. 1 log, you can use 1 splintered wood for every time you need fire, giving you the ability to. With the log, you have to use the entire thing if you want a fire. If you do a long crafting project (grinding flour with mortar and pestle which takes 1hr15mins), the log will burn up around I think 50%+. 50 splintered woods can do 1 project each meaning 50 projects, but 1 log can only do at most 5 projects.
If you’re talking about just light, then single logs are the way to go. They burn the slowest and provide consistent light. If you set multiple splintered woods on fire at once, they will give a more violent fire than the log (more fuel), but for shorter (they don’t last long).