You're kidding, right?
Knex is limited in color and shape to about 5 colors and 20 shapes.
Lego has about hundreds colors and thousands of brick shapes.
So colors matter? and if you are intuitive and creative, you don't need to look for that one piece that you need. With knex, you make that piece.
Make: Trebuchet, Piano playing robot, crossbows, bows, guns, (<-etc)
With legos, Make: a hauwse :D