on a technical side, lzma is a better choice. I might add a lzma recommendation, however, in soma cases lzma performs worse than bzip2.

also, decompression speeds matter, not compression speeds.

in the end, use what you prefer. the compression is not mandated by the spec. the main use for farbfeld is also rather thought to be as a piping format. it could also be used for storage, but that's not the main point.