I have seen this question posted a million times in all kinds of social media sites, so I am going to attempt to give it my best answer! Honestly,.....................................BOTH!
That is correct, there is its use for each in the papercrafting world. What do I mean? Well, let me show you!
2. Making a border- Now if what you want to ink is a paper border, that is another story :) I find it that a blending tool works faster and is more efficient giving it a sharper deeper look to your edge.
3. Lastly if you are trying to blend in the middle of your paper the winner without a doubt is the blending brush as it will not leave those ugly looking round edges that the tool sometimes do. I know we all try but I have to admit I am not a master on blending so more than often those show up on my work and they are so hard to hide!!
I hope this little tidbit helps you create beautiful work!!