I used to have a 17" Craftsman rear tine walk behind but self propelled tiller with the reverse rotation. It worked well. Then I had a Bush Hog brand tiller for the tractor with forward rotation. As has already been said, one will push you forward while the other will hold you back. Either one does a good job. My preference is the forward rotation because I figure if I hit a big root, or bricks (which I did once), the forward rotating tines will hit it on the downstroke, causing the tiller to jump up in the air and go on over. But with the reverse rotation, I'm afraid if it hits an immovable object on the upstroke, it just might break some tines, or hopefully just a shear pin. Of course some have a slip clutch that I never had or wanted so that night prevent damage.
So I reckon it boils down to personal preference. Either one is good.