I've used a Harris, an SLX and a Ninja.
I'd say the Harris is a good few feet shorter than a Ninja, and a lot heavier too. So the advantages of the Ninja are, it's got a longer reach and it's easier to use.
It's faster too; I quite like the Harris clamps; but the Ninja is better because it's clampless; you just extend, then give a little twist to tighten; it's firm; you clean the window and to shorten the pole it's really quick because it's just a little twist (like a quarter-of-an-inch; a tiny movement) and the pole section smoothly collapses.
It's easy over conservatories, and it's robust enough to 'tap' open windows closed.
One thing I think it can't do (though I haven't tried yet) is push up top sash windows. When faced with an open top sash, I'll use my SLX to push it up, so it's closed, but I don't think the Ninja could do this. That's the only negative I can think, but all the positives outweigh that.
Just make sure you stick a nice light brush on the end; I use a Gardiner's Superlight, and it really is a nice, rigid, light pole to use, at a nice price too.