Most of what has been said is generalities yet many have been stated as pure fact.
1. First and foremost MTL and DTL are inhalation types only. In general MTL specific devices do use really tight air flow, higher resistance coils, and higher nic juice and DTL in general uses much looser air flow and lower nic, they do NOT have to. I personally vape MTL style on "DTL" tanks using a restrictive 510 drip tip and slightly closed off air flow. This gives me a nice loose, easy to draw, MTL experience. I cannot stand trying to use "true MTL" tanks. If you like sucking fresh milkshakes through a coffee straw, then true MTL is for you.
2. Also the resistance bit, I do my MTL using TC mode with SS316L wire, you are never going to get that in a 1.2 or higher ohm coil. So 0.5 ohm is what I use. So saying that MTL uses 1.2+ ohm coils is again a generality. They often do, but it is not required.
3. Battery life is again a generality. Battery life is a function of mAh and coil resistance (as well as board efficiency and a couple other factors). With higher resistance coils you do usually use less power thus batteries last longer, but again it doesn't have to. One person may prefer a warmer MTL vape at a lower resistance and higher power than someone who prefers a cooler vape as DTL using a higher resistance and lower power. Number of coils matters too...a LOT.
4. There are numerous factors in actual nic absorption, and while it is true that in most circumstances nic content is higher in MTL devices, it doesn't have to be. I am doing MTL using only 6mg/ml and I did 3 for a long time, I just wanted a bit more. I did start at 18mg but that was on cig-a-likes with anemic clouds and super tight airflow. I dropped to 6 as soon as I got my first EVOD and haven't looked back.
5. As far as which one gets you more nic I'd say it depends on a LOT more factors than just MTL vs DTL. Just as a for instance, if you are making bigger clouds, that is more nic per puff so you may need fewer puffs than someone making smaller clouds using the same liquid. Other factors would include how long you hold it in, do you exhale fast or slow, etc.
6. Clouds. Yes a quad coil straight VG DTL tank is going to chuck some serious clouds. Way more than any single coil "MTL" device ever will. However given the same coil type and quantity and the same juice, they can still be similar. My wife and daughter both use an Innokin Ares tank (true MTL) and both blow some pretty anemic clouds with them. I can take their devices and without changing anything easily blow double or triple the cloud they do and I am not even taking a longer draw. I am not sure why this is, but how you draw (even before inhalation) seems to matter...a lot. Also how long you hold it in matters.
The point of all this is not to say that anything anyone said above was inherently wrong, just that there are exceptions to nearly every rule. The bottom line here is that there are near limitless combinations and you just have to find what works for you. Maybe you like a tight draw and MTL inhalation, maybe a loose draw and MTL inhalation, or maybe a really lose DTL inhalation. Whatever it is you need, it's out there, somewhere.
As to how to DTL, I find that it is much easier if you don't close your mouth too tightly and as you inhale directly to the lungs let a little air slip in from outside of the tank through your lips. Good luck.