As the owner of both opus and orizon
The opus, being a 5" is smaller, with the right cover (like this
http://www.bookeen.com/store/europe/...he-cybook.html), it's very handy to carry around. The opus can be in it's cover and still be held one handed.
The orizon is real thin even with the neoprene case. Didn't tried the other one. Tactile also brings improvements, notes, and other stuff.
They're both good devices. Guess the question there is : do you fell you might use annotations. And yeah, a 6" is a bit more comfortable.