Having used both, personally I highly preferred the Pebble over the Bangle JS.

Pebble was solid software, good designs and it all just worked simply and did what I needed. I also thought the Pebble Time Round was near perfect in design and execution. I'm not a fan of the geeky look of the normal pebble and bangle watches (or the apple watch look).

The Bangle was fun to dev for, and I love that it exists, but it all felt like a dev project. Not a finished product. Granted it was early on in the project so I'm sure it's in a better place now.

They both have similar capabilities. I say go with whichever you think is going to fit what you need a smartwatch to do.