In my experience, they're pretty comparable. Basically no real discernable difference in real world usage. Testing shows the 30Q ever-so-slightly beat out the HG2s. I vape at low wattages so battery sag is never really a big issue for me with either the HG2 or 30Q. But the HG2s are usually a little less expensive by a dollar or two, so 90% of the time I go with the HG2s unless there is a sale on 30Qs that make them about the same price or a little cheaper. On the flipside, the HG2s are more commonly counterfeited. If you go to a legit seller, you don't really have to worry about the counterfeits.
Edit: 30Qs have apparently gone down in price or HG2s have gone up in price, so at both Illumn and IMR the prices for the two batteries are the same so HG2s no longer have that going for them. Absent a sale the 30Qs are now the better buy.