Both systems can display rich dropdown content. The mega-dropdown is just the display from a Stack on your dashboard. The system shows one stack by columns (with capabilities for assembling blocks on each other for certain columns) and can display the block name optionally.
Add a Mega-menu under a page
Create a stack named with the handle of the root page. If you root page is "My Page" the handle would be "my-page" so the name of the stack will be "mega_menu_my-page" (note the difference between _ and -).
Then, add block to the stack. On each block you can optionally click and choose "Advanced" to name your block. This name can be otpionally displayed on top of the block, into the mega menu.
Each block is rendered in a new column. To stack different block down the previous, start the name of the block by "-follow-". In this case the block will be rendered UNDER the previous. So a block you want to call "Our Features" call it "-follow-Our Features" to place it under the previous.