Don't quote me, but as far as I understand, they're kind of somewhere in between. I don't think they're capable of serving as general purpose computers the way a Pi can, but much more powerful than Arduino. I believe they're designed for much more advanced IoT prototyping for industry, whereas I'm pretty sure Arduinos are aimed squarely at makers and DIY enthusiasts.
I could be totally wrong, but this is the impression I got when I took a look at them hoping they could be used like a Pi to create a tiny, neat PC. Hopefully someone who knows for sure can chip in!