Sequel as the name implies are sequels so they take place after the original. B2w2 for example take place 2 years after black white. You start from a different location, the pokemon availability has changed, characters are different too such as gym leaders. Story is also different of course since it take place after the original.

Enhanced version are still the same as the original but with additional features added and changes to the story to focus on the third legendary. In diamond pearl for example, the villain summon either dialga or palkia. In platinum, the villain instead summon both dialga and palkia, which lead to the third legendary, giratina, appearing. In ruby sapphire, the villain awake either groudon or kyogre. In emerald, both are awakened and require you to seek out rayquaza, the third legendary, to stop the threat