1. Kellog's has a corn-flakey bran flake texture, while Post is crunchy and has a more substantial mouth feel
2. The raisin to bran flake ratio is more favorable (although the raisins in Post are slightly inferior, too sugary)
3. In my experience, the digestive effects of Post are more desirable (subjective)
4. Due to the composition of the bran flakes in Post it gets soggy less quickly
5. Price. This might vary by location, but at Winco and Walmart I'm usually able to get Post at a lower price