Unless you have a specific use for the 28mm f/1.8, I'd probably skip it because you have the 16-35LII. There's only a 1 1/3 stop difference between the two lenses, which is not usually enough for me to sacrifice the versatility of the L zoom for the prime.
Having said that, I have the 16-35L and the 28mm f/1.8. I've also owned the 35L and 24LII. I chose to sell of the L primes and in the end go with the 28/1.8 because of it's small size and accurate AF. I use it specifically for street and photojournalism and find the 35L to be too close to my 50/1.4 in FL and 24LII was too big and (oddly enough) not as good at locking AF in very low light as the 28. It's not the sharpest tool in the box, but it is more than usable from wide open. It really shines on FF imo.
Good luck in your decision.