1) more volts can get more heat out of the same atomizer or cartomizer. More heat is more vapor and hotter hit. In practice - you have a single voltage battery and you put lower or higher resistance (ohms) attachments on it to get the warmth or heat you want (this is applicable to FAT batteries and mods since slim ecigs should only use standard resistance). However, there is a limit of how low resistance an atomizer or cartomizer you can put on any particular voltage or it will just pop or singe. So practically speaking there is less difference than you would think. On any voltage, some resistances will be considered standard res and be warm and some lower ohms will be consider LR and be hot. Like a 3.0 ohm atty on a 3.4v battery will be barely warm but a 3 ohm atty on a 6V battery will be very hot. A 1.8 ohm atty on 3.5v battery will be hot but if you put it on a 6v battery it will just pop.
In the world of slim ecigs, I think the hottest is KR808D-1 (3.7v slim mini) with cartos around 2.5 ohms (like Vapor4Life WOW cartos, and Bloog/Volt cartos come close but not as hot with that bottom coil)
2) variable lets you apply different voltage to the same atomizer or cartomizer. So you can crank up the heat to your desired level, within reason (go too far for any particular ohms atty or carto and you may fry it)
3) So far? I have nothing exotic or high voltage. For me any real 3.7v battery. KR808D-1 for the slim ones, Riva 510 from Liberty-Flights.com (their kits are now 3.7v), 3.7v mod boxes (each uses one 14500), the 3-stage clone Ego (3.2, 3,7, 4,2v). But honorable mention to the Joye 510 mega xl manual battery which is just darn nice with a Boge 3 ohm carto - just warm, vapory and pleasant w/ nice flavor.
