spade bit V hole saw

This compares spade bits to other types of wood drilling bits, including hole saws:

If you wanted a deeper hole. Hole saws as used by amateurs are usually no more than about 5 cm long. Some might do 8 cm. A 20 mm spade bit could make a hole of 10 or 12 cm. Conversely, AFAIAA, spade bits are only for wood. Hole saws can do metal or hard stone, particularly if diamond-tipped.

For drilling a 20mm hole in wood, I wouldn't use either - I'd use an auger bit.

When would you use a spade bit/hole saw?

They are good for flat bottom stopped holes, but there are better tools for most applications.

Flat bottom? All spade bits I have ever seen have a 'pointy bit at the front' surely if you drill a stopped hole with these you will get a large spiky hole in the middle.

So long as it does not penetrate right though the wood, that usually is not a problem.

I use a hole saw for cutting fairly large holes in sheet material - such as cutting holes in ceilings for downlights.

I have in the past used spade bits for cutting cable holes through joists - but they cause a lot of vibration and don't make a very clean exit. I very much prefer an auger as long as there's enough room to be able to us it. In tight spaces I may occasionally use a spade bit, cut off short, in a right-angle drill.

Where possibly, as soon as the spike end appears through the far side, I swap to drilling from that side to leave it clean both sides.

Above a certain size, I switch to an adjustable thing like a double-ended tank-cutter.

Spade bits are cheap and somewhat disposable, and have an ability to drill non through holes, and also very deep holes (with extension bars, in excess of a metre is possible).

Hole saws convert less material to "dust" and are more "efficient" - especially in hard materials and metals where a spade bit is not much use. They are however harder to use, and slower - especially where you need to remove the plug of material from the saw after each hole. They are also more expensive.

So I would use a hole saw typically for larger diameters and in thin materials.

(one probably ought to include augers, expansive bits, forstner bits, annular cutters, tank cutters etc into consideration for larger holes - each have pros and cons)

I have a box of "stubby" augers that work well for that... Also easier to drive in confined spaces since they pull themselves through without needing to apply pressure to the back of the drill.