I kind of have to reply to this thread as I've gotten my first mechanical mod a few weeks ago....
I've been vaping on variable wattage/voltage devices ever since I upgraded from the standard affair starter kit. They allow me to better adjust the wattage needed for the different coils I have been making myself. I've done a bunch of twisted, clapton, and just about everything that involves round wire (haven't bought any ribbon yet.) Problem being, I haven't gotten a variable wattage device that is particularly strong yet (been using 40w max.) At 40w, all those twisted wires, clapton wires, and everything in-between has a difficult time heating up fast and giving me that good vapor. However, I can tell you that temperature control with Ni200, 40w is plenty sufficient for that usage.
On mechanical, I've got a Tesla Two which isn't the best by any means, but it does very well for .1-1.7 ohm builds. I was using a clapton wrapped 24 gauge with 30 gauge kanthal dual coil setup 3.5 wraps and it performs very well. I just switched it to dual 7.5 Nichrome 80 22 gauge and is performing just as well. Mechanical is great for low-ohm builds is generally what it comes down to.
Now if you're looking for a comparison of which to buy, well, the answer really is what do you want it to do. Obviously, variable wattage devices allow you to tweak the settings of wattage to better fit how you vape, and allow for better control of those different style coils. Mechanical mods on the other hand, do just what they're supposed to, and that is to supply voltage to the coil(s). What you need to have in mind if you're going to purchase one and go all out is, having many different options to choose from is a definite perk. In this general idea, many options including temperature control and whatever else you can get, the variable wattage devices are basically the only thing you should be working with. On the other hand, if you just want to hit the fire button, get a quick vape or two off, mechanical devices work very well for that. The basic knowledge of it is, variable wattage devices have a computer chip that controls the voltage/wattage and thus allows vapers who like to tweak their settings and change their vaping style a lot to do so. Mechanical mods are you have to change the coil to get a different style of vape.