For the vast majority of
online poker, that I played, I have had stacks displayed in chips (or dollars for cash games), but my HUD has shown the number of big blinds. So I guess my answer is "both". Lately though I have experimented with showing stacks in BB in tournaments, my main reason being, that the HUD always takes a hand to update to new blinds, every time they go up. And this has caused me to many times play a hand a different way, than I would, had I known, that stacks were now shorter than shown on the HUD. Knowing the amount of chips is nessesary though in bounty tournaments. But on both
pokerstars and ACR its very easy to click the display back and forth between BB and chips. So for now I have just settled on doing that, when a bounty will be in play, and I have a close decision.