Plastic partial dentures are relatively easy and inexpensive to make. They are most suitable when quite a large number of teeth are missing.
Metal partial dentures are much stronger than plastic and therefore can often be made much smaller and thinner. Being more intricate to design and manufacture, they are usually more expensive.
The dentist will advise you as to which types are available, and which will be most suitable for your mouth. Factors which influence your decision may be cost, the final appearance, the amount of your palate to be covered by an upper denture, and the size you know can tolerate from previous experience.