What is the difference between a Bowden and Direct Hotend?
This article does a great job of explaining the key concepts of what a Bowden extruder is and why you may or may not want one compared to a Direct extruder: http://www.fabbaloo.com/blog/2015/11/11/bowden-or-direct-a-primer-on-extruder-styles
In terms of E3D-v6 hotends specifically:
- All 1.75mm E3D-v6 HotEnds are the same physical parts. However the bowden kit is supplied with a longer length of tubing, and an additional threaded bowden coupler for connecting the tubing to your extruder drive system.
- 3mm Direct and 3mm Bowden E3D-v6 HotEnds use different HeatSinks. The 3mm Bowden HeatSink has an integrated bowden coupling, and the Bowden kit is supplied with tubing and a threaded coupler. The Direct kit has a HeatSink with no integrated coupling, and is supplied without any tubing.
What are the differences between 12v and 24v HotEnds?
Almost all printers use a power supply that powers things like the motors, electronics, hotend etc. These tend to output either 12v or 24v from which all components in the system run. This is unrelated to localised mains voltages (110V/240V AC).
You need to order a kit which is the same voltage as the internal voltage of your printer. You can usually find this information online, or written on the power supply of the printer.
E3D HotEnd full kits are supplied with cartridge heaters and cooling fans. These two components must be matched to the supply voltage of your 3D printer system for safe and reliable operation. If your printer has a 24v power supply output then you need the 24v HotEnd kit, likewise with 12v power supplies and 12v kits.
Thermistors are not voltage specific components, and so it does not matter what voltage your system uses, you should be fine with the single thermistor option.
There is no performance difference between 24v and 12v kits.
E3D-v6 Specific Questions
The E3D-v6 as sold is limited in temperature by the thermistor to 295*C but can go higher (tested up to 420*C) with a thermocouple upgrade.