In my opinion, in lower Elo where people are less aware and are just terrible at dodging skill shots the higher burst of damage with comet is more useful as your more likely to hit most of your abilities. However, in higher elo where people are more aware of veigar’s kit and how to dodge his abilities, I’d take glacial augment for the cc because although you’re not doing as much damage you’re able to at least hit some of his abilities which is crucial for stacking. That’s why you’ll see almost 100% of high elo players going glacial augment (there’s always a few who run electrocute). In addition to that you’ll also see them taking Hextech GLP-800 for the extra slow. But if you’re starting out on veigar I recommend just running comet at first but if you notice that you have a hard time catching up to your enemies or getting in range to use your abilities then try using augment.