For Pet grooming, trimmer is used mainly for hard to reach areas such as face, ear and paws. Characteristics of trimmers: slim, less bulkier, lighter, shorter blade width & powerful motor.
Hence, increase precision control for hard to reach areas as the characteristics of trimmer would allow better handling. If you are looking for equipment used for trimming areas such as face, ear, paws or sanitary area, trimmer is highly recommended.
Clipper is more for heavy duty usage and use for body clipping. However, if needed, clippers can be also used on delicate areas. Characteristics of clipper: Can be slim or slightly bulkier, wider blade width & powerful motor. Compare clipper to trimmer, clipper would require better handling on difficult areas, precision wise, trimmer would be a much better choice for beginners. However, If you have the need to clip your dog or cat body, the wider blade width of clipper would be a great help. Of course, the higher RPM clipper would have a smoother clipping for the body.