If the Verbatims you see are Made In Taiwan, and have "AZO" on the packaging, grab them. Greedily. With both hands
Verbatim + Taiyo Yuden FTW!
As for HP DVD media - you may have hit on something there: many media sellers simply put their name on the products of different companies. Your HPs might be a mix of CMC MAG and other media products where the manufacture was "outsourced" and the remote maufacturer simply but the branders name on their own discs.
It's quite possible that the reason your HPs burn at differing speeds is because the discs are of differing disc models. Most here don't burn DVDs at 16x anyway, and recommend limiting burn speeds to 12x, 8x, some even 6x.