They are 2 completely different devices.
The variable voltage or wattage device is a true electronic cigarette in that it actually contains some electronics inside of it. It let's you adjust the voltage of your battery to make your coil get hotter or cooler. They usually have built in protection that will shut off your device if it sense that something is going wrong with your battery or your coil... Overheating or shorting for example. They are also regulated. This means that as your battery looses charge as you use is, it will boost the signal of your batterie to always match the voltage that you have it set to. Let's say you entered 3.7 volts... The Variable votlage or wattage device will give you a constant 3.7 volt throughout your vape time until the battery dies...
The mechanical mod on the other hand offers none of this. It is an electric device that contains no electronics. They use a mechanical switch that connects the positive and negative pole of the battery physically with the use of springs and moving parts thus the appelation mechanical.
They contain no builtin protection whatsoever that will prevent your battery to go berserk. The only protection they have will kick in only are after the fact that your battery died, blew up or melted. There is no way to adjust the voltage of the device without the use of added electronics... You are thus vaping off directly from the battery which means as the battery drains itself, the voltage will drop with it... You will start out vaping around 4 volts and slowly but surley end up vaping on some 3.4 volts.
The mechanicals requiers more knowledge about battery safety to use properly. Nothing will prevent you from overdischarging your battery, and a overdischarge battery can lead to serious consequences, like fire or explosion. They are not performing well with regular atomizers, clearomizer, cartos and so on... They are recommended for the more advanced users that wants good performance out of rebuildable atomizers. But a lot of research is needed to know how to operate them safely.