Spirals are more efficient at cutting channels but it depends on what you are trying to achieve, down cutters are perfect for channels where the top edges need to be crisp, these must be done in small increments because the shavings will be pushed to the bottom of the channel and back up against the cutter, it then needs to be forced with devastating results, two passes will clear the rubbish. Up cutters will clear the rubbish as you cut but may chip out the edge so use for channels where the edge is not visible. Because of the unique cutting action, the feed rate of your work must be done faster than with ordinary cutters, too slow and it blunts your cutter quicker but again due to the slicing action of the spiral edge these cutters rarely need sharpening but again that is also down to how you look after your cutters, rout dirty gritty wood which has been dumped on the floor of your workshop and the most expensive cutter will suffer.
Because the whole of the spiral is in effect the cutter and usually made from carbide steel, it is more brittle and if used incorrectly will self destruct, any attempt at climb cutting must be made taking the thinnest possible cut and I would hold the work very tightly and feed very slowly.
The other thing about spirals is that because of the cutting action the tendency to vibrate is virtually removed but again that can be relieved on straight cutters by taking thinner cuts.
There are straight cutters available with a shear cut, the TCT blade is set at a slight angle and these will leave a very smooth edge on boards, also the spirals are good for plunge cutting but again straight cutters can be obtained designed for plunging with cutting tips built into the end.
The deciding factor is usually cost.
Andy
