Straight knitting needles made of wood (including bamboo or hardwoods) offer a higher level of friction when in contact with the yarn. This means that the stitches slide more slowly, providing extra control, which is especially useful when working with very soft yarns such as cotton, merino or alpaca. They also have a warmer, quieter texture and are ideal for those who prefer a more leisurely and tactile knitting experience.
On the other hand, metal needles, especially aluminium or nickel-plated steel, allow the stitches to slide more smoothly. They are perfect for fast knitters, those who work on complex patterns or with yarns that tend to snag. However, they can be more slippery and cold to the touch, which is not to everyone's liking.
The choice will depend on the type of yarn, the pattern, the knitting technique and personal style. There is no best type, only the most suitable one for each project.