When you need to make a straight cut, such as a dado or rabbet, should you choose a spiral-cutting router bit or a straight-fluted bit (with no twist)? Which one is better? Well, I guess that depends on your definition of "better," because there are distinct advantages to both.
Spiral-cutting bits come in two varieties, up-cutting bits (meaning that the bit's motion lifts the cut debris out of the groove) or down-cutting bits (debris is forced downward). Both leave a clean cut, but where down-cutting bits tend to leave a very clean top edge but a rough bottom, an up-cutting bit leaves a clean bottom but somewhat rougher top edge. When working with a down-cutting bit, you'll need to proceed a bit slower, because you'll need to wait for the debris to clear the bit. An upward cutting bit doesn't have this problem but has a tendency to lift the workpiece if it isn't securely clamped to the work table.
In either case, the spiral-cutting bits tend to cut a bit more efficiently with less vibration than a straight-cutting bit. They also excel in plunge routing applications, due to the drill bit-like motion of the cutter. However, they are considerably more expensive than straight-cutting bits, and they aren't available in nearly as many sizes.
On the other hand, straight-cutting bits have advantages over their spiral-cutting cousins. First of all, they're less expensive and come in many more sizes. They are readily available at many home centers and lumber yards, where spiral bits may be a bit more difficult to find. The straight bits are much easier to sharpen as well. Additionally, if your application requires the use of a guide bearing on either the shank or the tip, you're pretty much restricted to using straight-cutting bits.
The disadvantage of a straight-cutting bit comes into play when you want to plunge the bit into the work. Because the cutting carbides do not extend all the way to the center of the bit, plunging directly downward into the work isn't possible. Instead, you'll need to plunge downward about 1/8" and move along your path, then make another pass another 1/8" deeper until the desired depth is achieved.
Also, straight-cutting bits tend to heat up a bit faster than their spiral cousins, which can cause a variety of problems, not the least of which is burning the wood, which will require you to clean up the burns from the cut before finishing. That is never a great option. In addition, excessive heating of the bit can cause the metal to lose temper, which is a technical way of saying that the bits won't be able to hold their sharp edge on the carbide cutting surface for nearly as long. However, if you move through the wood at a moderate speed and don't allow the bit to stagnate in one spot for long, thus keeping the heat to a minimum, your bits should last longer and hold an edge much better.
So, back to our original question: which type is better? As we've seen, it really depends on how you need to use the bit.