FCAW/flux-core arc welding burns a bit hotter than MIG/Metal Inert Gas welding and is better for welding thicker material, especially when outside and/or dirty material ... with that said, I like to clean the surfaces to be welded to reduce spatter and porosity and if the steel is left out in the cooler mornings with moisture present (Dew, rain, etc.), I like to warm the surfaces to burn-off the moisture (if possible) to again reduce porosity and spatter. FCAW also leaves slag, like stick welding, that needs to be removed before paint, etc. I use flux-core often to reduce costs, but clean and warm the surfaces when possible to reduce porosity and spatter.
On the other hand, the inert gas used in MIG welding offers a bit better shielding cover when used inside, out of the wind and leaves a cleaner weld without slag to clean off.
As far as Inert gas selection:
--Argon, Helium, Carbon Dioxide and Oxygen are the four most common shielding gases used in MIG welding ... A good basic gas/mix depends on the metal to be welded, the penetration, and clean-up ... For example:
--CO2 is good for penetrating welds on steel, but may be too hot for thin metal ... For thinner steels, use 75% Argon/25% CO2.
--Use only Argon for aluminum. You can use a triple-mix for stainless steels (Helium + Argon + CO2).
The welder should come with a user's manual that will give similar uses and offers the manufacturer's suggestions ... One can play with the different mixtures to change the penetration and puddle shape/size, and clean-up; however, it is probably best to follow the welder instruction manual at first and of course, there is a ton of info online.
I apologize for the loooong winded answer, but hope this helps you get started ... good luck.
goind to buy the 
