Hi,
on the management side, ART has a much shorter release cycle.
On the development side, RT takes backwards compatibility very seriously. This is clearly very important, but that means you have to carry around a lot of legacy functionality. With art I wanted more of a fresh start. Also, local editing is handled in very different ways by the two apps.
Finally, the UIs are superficially similar, but RT is significantly more geared towards power users, offering a lot of control over quite a bit of low-level parameters. In this respect it is similar to darktable. ART is more targeted to “average users” (whatever that means…), and each tool in the UI tries to have fewer simpler controls that try to do “the right thing”. Whether or not this actually works is a different story, but that’s the goal 
HTH