It's hard to say. Each has its own advantages.
Workload: electronic materials are more complicated.
Payment: electronic material payment cycle should be short, easy to operate.
Difficulty of work: the mechanical part is more difficult, but the supplier is easy to bind. In fact, your question is very vague, because we do not know your age, experience, and career plans.
I personally think that men, I think it is better to structural parts, but there is no experience in electronic materials procurement, you can choose electronic materials first, because now many companies are required to have electronic materials procurement experience, and your future goal will definitely be the procurement director or manager, do not understand electronic materials, it is a little difficult, but as a man, I think structural parts are more promising than electronic materials, look at the job requirements.
Choosing which field to do is inseparable from your career plan, and if you want to stay with that company for a long time, choosing an organization is the right thing to do. Generally speaking, it is very promising.