the choice of tool material and its coating/uncoated, depends on several factors. basically the tool needs a good ballence between hardness, abrasion resistance, ability to keep its edge when heated and sharpness. to look at each area in turn; hardness is the basic ingreadiant...the tool needs to be harder that the work, but too hard the tool is brittle so toughness also comes into it. closeley related to hardness is abrasion resistance (a hard material is not always that abrasion resistant, a pencil is a good example) coatings being >3000vpn give a higher surface hardness. hot hardness comes next...because as the tool cuts it generates heat... the ability to withstand this heat or ability to not absorb it by some means is a real bonus...coolant unless copiously applied can have a negative effect on the cutter, whilst the coolant removes the bulk of the heat, if it's not applied in sufficiant volume the coolant boils at the tool cutting edge causing thermal shock and cracking in the cutting edge microstructure, some of the latest coatings lower friction (e.g. Zirconium)and thus lower the overall heat build up. sharpness plays more than a minor role in cutter design and affect all the previous, basically for a new tool, a sintered carbide insert with a PVD coating i as blunt as you'll get for new, let me explain...inserts are made by powder metalergy, they are isostatically pressed in a mould and then heated to fuse togeter all the particals... if you look at the insert edge under a 20x lens you'll see that is kinda rounded off i.e. BLUNT, now if this is then coated its like wraping cardboard over the corner of a table... the card corner is even more rounded!! a blunt tool generates more heat and more rubbing (abrasion) so on some levels the cutter design is self defeating. to counter this effect ground inserts can be aquired, these are often ground using a D150 diamond wheel, but under that 20x lens you would still see a very rough edge, hear is where HSS is better... it can be honed to a keener edge (think of the barber who sharpens his cut throat razor on that leather strop)but HSS has not got the benifit of hot hardness that carbide has... a good compromise hear is the often overlooked ultra high cobalt tool steels (i.e. 30-40% Co)such as Tantung or Stelite...anyway after all this ranting my final conclusion is...look at the cutter manufactures data... use the cutter they recomend for your application i.e. take into account if it's a roughing operation, intermitant cutting, finnishing cut, material type etc...don't forget if you use coolant...use pleanty of it...also follow the recomendations on speeds and feeds (don't be scared)but also take into account that they will have tested thier cutters on the best possible machines under the best conditions so they can publish bigger numbers than thier competitors so go for 30-50% less and creep towards thier recomendations. stick with the better known brands and if the data is not available you can always get it from another book and go a bit less than recommended