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.
To offer another take, FF has never been buggy for me. Never had this keyboard issue, or any of the stuff mentioned so your mileage may vary.
As for the bottom nav, I have that on normal FF. If you go to Settings–>Customize, the option to pick top or bottom address bar is right there.