Plunge bits are different from other straight bits. The end of the bit has cutting edges to bite into the wood as it is lowered. The less wood you are actually cutting means the easier the job, so a 1/4" bit is what I use. This has the added advantage that 1/4" diameter bits are less expensive than larger sizes. Spiral cutting straight bits tend to make a better finish cut than a simple straight bit, more of a slicing action as opposed to a scraping action. They also help eliminate the slight burns, bumps and tear out that can occur at the end of a circular cut. The spiral bits are either up or down cutting, depending on which way you want to eject the chips. Remember to make your cuts in small passes, ie... no more than a 1/4" depth at a time. Again, the less wood you are cutting means an easier cleaner cut.