Hi Zee - I think you are referring to the difference between the round and the rectangular? where as I was referring to two different round inserts - plain and TiN Coated.
I've not been a carbide user at home so these are new to me (in the home workshop) and it's a good eighteen years or so since I last used anything like it so - 'laymans' description
The round 'tips' give a much better surface finish but with with a much less depth of cut than can be taken with the rectangular type. The finish off the rectangular, whilst acceptable enough is not comparable to that off the round.
Basically then - rectangular for metal removal, round for good surface finish
If it's the difference between the plain and coated - the coated will usually last longer or can be driven a bit harder than the plain
Tug