There is an ad-hoc comparison of a few of top existing services here:

http://imagga.com/blog/imagga-and-6-alternative-image-recogn...

and here: http://imagga.com/blog/some-more-image-recognition-tests-som...

Disclaimer: I'm CEO & CTO of Imagga.

It would be very interesting if the two looped together, each using the other as a reference to improve their own parameters ;).