How is block different from mini panel?
Mini panels allows you to use layouts, so you can have rows and columns within a mini panel block. Making life way easier when it comes to applying css. 

In addition, it allows you to attach other things, such as views, other blocks, etc.. 
In contrast, if you use a regular block, you can only do one thing at a time with them. 

So if you have lots of blocks, it can be messy if you want to change the block position. While if all the blocks are inside a mini panel, you can move them all at once. 

Don't think of it as mini panel vs block. Think of it as mini panel being a block management system.