From the wording of your question, I would have to ask why you want to vape specifically at 6v+? "It's all about the watts" is a commonly repeated phrase round these parts

Voltage is just one part of the equation of Watt's law, and the related Ohm's law. The wattage (power) will determine the heat of the coil (and the warmness of the vapour) and not just the voltage in isolation.
As you point out, vendors often now give you the resistance of carto's and atty's in Ohms (Ω). This bit of information determines how much power (watts) the coil outputs for a given voltage. An Ohm's law calculator like
this one allows you to play about with different inputs without doing the calculations yourself.
To explain, a 2 ohm carto (like the common Boge LR 510 carto) when run at 4 volts will theoretically output 8 watts. A Boge 3 ohm carto, when run at 4.9 volts will also output 8 watts. Which will be the best to vape on? Neither - they should be exactly the same. If you had a fixed volt device at 6v and you wanted to run around 8 watts on it, you would need to look for a 4.5 ohm atty or carto.
It has been said that the "sweet spot" is around 8-10 watts. Personally I am finding I use in the 6-11w range, depending on the e-liquid (high power murders some flavours) and there are other factors than just this at work - for example CE2's thrive on a lower wattage than the Boge's in my experience.
The advantage of variable voltage is that you don't have to be too picky about which cartomizer/atty you use. You should be able to dial in to the "sweet spot" regardless of what you use.