http://www.creativefieldrecording.com/2014/02/05/izotope-rx2-vs-rx3-whats-changed/Basic version upgrade: faster, easier interface, added normalize gain function, plug in functionality, dithering and resampling added. Not sure if improvement to have more automated the denoise function in RX3 basic over RX2.
RX3advanced gets: declip, channel operations, denoiser has more manual control, dialogue denoiser, dereverb and free insight tools.
A couple of posts on GS in the post production forum are favorable towards the basic upgrade, but seem much more enthused about the advanced product, especially at the price through May 1.
If I was paid to work with recorded dialogue on a daily basis that I had to clean up by removing bird chirps, vehicle noise, etc. like those guys, I could see why they'd be more interested in the advanced product.