6 ohms will draw more power than 8 ohms, but impedance ratings are at best a general guide. The actual impdeance of a speaker varies drastically across the speakers operating range. The impedance specification can only be used as a very rough guide to how much power a speaker will draw. The impedance rating is more useful as a guide to matching the load of the speaker to the amplifier - some amplifiers are not happy with 4 ohm speakers for example.

I wouldn't worry about this aspect, as you can go crazy with all the variables and different ways of presenting specifications. The important thing is that most HT speakers and amplifiers are compatable with each other - manufacturers have seen to that.