Does not really make a big difference both are easy loads on amps . A 8 ohm specified speaker can drop to 3 ohms at certain frequencies and same goes for the 6ohm speaker as well. There are far more important aspects to consider when choosing a speaker than just 6 ohm and 8ohm load
Normally the manufatures specify nominal impedence only in the specification.
How can we know the impedence drop..Only a freq v/s impedence graph can show the dip in the impedence right? If the imependece drops to a lower value say 3ohm, isn't it going to spil the amp if it is not able to push the high current needed at such low ohms.
Except for some speakers like the Magnepan etc., most speakers have a fairly variable impedance with frequency. This information is not given with the user manual or on most review sites. Stereophile is one among the few that do provide this information.
"Typical "" impedance is usually the impedance at a low frequency . 6 ohms would mean that it doesn't go below 6 ohms at 'some' lower frequency !What happens at high frequencies is not mentioned. Sometimes this is of no consequence but at other times it does matter as it does drop very low and stresses the amp.
At low volume this shouldn't matter much but as you increase the volume it can cause different types of problems. Heating up of the amp or blowing fuses or components. Often before all that it will start to sound worse and strained. That's when you have to turn back the volume.
I've attached the impedance plot of three speakers .
Dynaudio Excite X14 rated as 8 ohms in the spec sheet.
The other one is an electrostatic speaker Acoustat Spectra 1100 .
The impedance is rated at 6 ohms nominal ( but they do say 2 ohms minimal !) Not all brands say that.
The Wharfedale Jade speaker is rated at 6 ohms but will be a difficult load for an average amp !Look at the impedance graph. It goes below 4 ohms !