Remarkable

What screen size is your Boox? And is there a particular model you'd recommend?

I have a note 2 (4 years old devices, but still functional) that has a 10 inches screen, and I would not recommend to get something smaller. They have also the maxi series that is about 13 inches, that seems amazing if you don't have to carry it with you everyday. I would say any model like note 3,note air or newer would be good (not much differences between them Imo, just some small hardware upgrades).

How do you integrate with you workstation and is it easy enough to get the papers into the Boox?

Boox devices run with an heavily modified version of android, so I just create a shared folder between my laptop and the note and downloaded an app to sync them.
There should be also an official desktop app for that, but its servers are based in China, so synchronization was painfully slow for me (from EU).
It is not difficult to move stuff, but rm2 is definitely more user friendly.

Which device offers the best annotation experience, in your opinion?

Onyx has hundreds of features and it is far more customizable, but the sheer amount of options/menu is, again, not very user friendly. A very cool features of the rm2 is the capability to add blank pages to a pdf, this is something you don't have on note devices. For the hardware, they seems to have 2 different type of glasses on the display, the writing experience on the rm2 feels better, but that is subjective.

Trying to summerize, is like 10 years ago apple vs android, rm2 has just some features, all very nice and done with a lot of attention to the details, while boox devices does a lot more stuff, but in a more "nerdy" way.