Brain Octane assists with cognitive function with some metabolic effects, and XCT is more metabolic with some cognitive effects. For those interested in a cognitive boost and easiest digestion, Brain Octane Oil is a great choice. For those looking to economize while getting a metabolic boost with a smaller cognitive effect, XCT will do. Some people mix different ratios of XCT Oil and Brain Octane Oil.
There is also a Bulletproof radio episode you can check out here: https://youtu.be/3D8K3oX4_0U?t=1m50s where Dave also goes into more detail on the difference between different MCT's, as well as an article available here with more extensive info: https://blog.bulletproof.com/what-is-mct-oil-vs-coconut-oil/
and there's a great article with a definitive guide to MCTs available here as well: