I think the block characters are easier to read, but with a 4x4 grid the resolution is lower: https://en.wikipedia.org/wiki/Block_Elements

The resulting pixel resolution is a bit screwy, especially if you use a tall, thin font like the one I tend to. Too many pixels horizontally, too few vertically. The upper block/lower block characters might be better in terms of aspect ratio, but then the overall resolution is made even worse...

The 2x3 Braille grid makes a lot more sense.