Neither of them are well-designed or well thought-through. They address some cosmetic issues of Protobuf, but don't really deal with the major issues. So, you could say they are slightly better, but the original was bad enough to completely disqualify it either.

Flatbuffers lets you directly mmap from disk, that trick alone makes it really good for use cases that can take advantage of it(fast access of read-only data). If you're clever enough to tune the ordering of fields you can give it good cache locality and really make it fly.

We used to store animation data in mmaped flatbuffers at a previous gig and it worked really well. Kernel would happily prefetch on access and page out under pressure, we could have 10s of MBs of animation data and only pay a couple hundred kb based on access patterns.