Producing proteins in the lab (often in bacteria) is pretty routine work. However, producing a posttranslationally modified protein can be much more difficult, because you generally have to have access to an enzyme that will perform the desired modification. On the other hand, it is generally much simpler to mutate the source DNA, and then use that DNA to produce a modified protein (generally with one amino acid substituted for another)
Routine? Simpler to mutate?