It's much much smaller (I think libfirm is over 100kloc, QBE is about 6k).
But the major difference is the IL: I use a human-readable and easily-printable text IL. This means that you don't need a graph-viewing tool to read the IL (it's just text) and that you can modify the IL between two passes super easily. This simple IL is a blessing when debugging a compiler.
I think QBE also has better support for the x64 ABI.
Finally, it is much less advanced (less optimizations, less tested) than libfirm and supports only x64 as a target.
But the major difference is the IL: I use a human-readable and easily-printable text IL. This means that you don't need a graph-viewing tool to read the IL (it's just text) and that you can modify the IL between two passes super easily. This simple IL is a blessing when debugging a compiler.
I think QBE also has better support for the x64 ABI.
Finally, it is much less advanced (less optimizations, less tested) than libfirm and supports only x64 as a target.
This is a sketchy comparison.