A 1/2" shank is 4-times stronger and is "stiffer" than a 1/4" shank, so it has less wobble for a given sized cutter.
As for the better cutter diameter, I only have my own experience to draw upon, and that pales to many here but the larger bits seem to run cooler (for a given depth cut) than a small bit. I've wondered if it was because (even at the same RPM and with a shallow edge cut) the cutter moves further in the air, so experiences more (faster) room air. This is just my guess but I've noticed my 1/4" cutter bits seem to dull faster than my 1/2" cutter bits. Again, this is just my perception, and could be totally incorrect.
How about it guys... is it just my imagination? <g>
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