The thing about Klon(e)s is they do serve as a clean boost (meaning no OD at all) with some EQ shaping, unlike Tube Screamers which even with the gain on 0, are a little dirty.
I like TS's better for the stuff that I play, but looking at it as objective as I can, Klons are "better". They've got way more output volume, more range in the gain knob, a more innovative way of blending the clean with the distorted signal, a more transparent EQ that still helps boost things up for leads, way less compression, etc.
Of course, "better" isn't always better, really, LOL.