MIG welders: How do you choose?

MIG welders are an excellent fabrication and repair tool. Mig welding is an arc welding process that joins metals by heating them with an arc. The arc is between a continuously fed filler metal (consumable) electrode and the metal.
Mig welders & mig weldingThere are many mig welders models available on the market, but which is best for you?

A world of choices
Externally supplied gas or gas mixtures provide mig welder shielding. Common MIG welding is referred to as short circuit transfer. Metal is deposited only when the wire actually touches the work. No metal is transferred across the arc during mig welding. Another method of MIG welding, spray transfer moves a stream of tiny molten droplets across the arc from the electrode to the weld puddle. Consumables: contact tips, shielding gas, welding wire.
A number of configurations
Mig welder manufacturers include: Lincoln, Miller, Hobart, Century, Esab, Clarke, and others.
These mig welder manufacturers offer a variety of mig welders that can meet any welding need from industrial mig welders to hobbyists to fabrication shops, ranches, aerospace welding, and more.
Terminology & explanations
Mig welders utilize solid wire and require shielding
MIG welders use a solid wire and require an inert gas for shielding. Flux-core welders use a hollow wire that has flux inside it. When heated, the flux forms a protective gas layer around the weld. The shielding is there to prevent air from getting to the weld and causing a variety of problems, such as oxidation to hydrogen embrittlement from oxygen and water vapor in the air.
n mig welding, you can also use an inert gas to shield flux core - doubling the protection.
The wire size has more to do with the size of the welder as anything else. The cheaper 110 volt welders probably can't power a much bigger wire, so .030 tends to be more common. You will need larger wires to weld thicker gage materials. For a 110 volt machine, the maximum wire size is probably around .035".
Flux core is good outdoors because a breeze will not blow away the shielding gas/slag. Mig is good for aluminum and stainless.
My advice would be to get a dual purpose machine that can do either Mig welder or Flux core, and size it according to the gage of material you intend to be welding, and also to your expected available power supply.
Mig or Tig Welding?
I've never tried to weld stainless with anything but TIG.
It uses a power supply similar to a stick welder, but instead of a consumable electrode it uses an non-consumable tungsten electrode. It is also similar to oxy-fuel welding in that one must provide filler material with a separate rod. TIG is good for difficult to weld materials, such as aluminum or stainless steel. Everything else works better in mig welders, which offer you a world of options for a good variety of materials.
Mig or flux core?
If you are going to do a lot of outside welding on steel, then you'll be using more flux-core wire. Inside, MIG welders may be better, and it's the only choice on that machine for aluminum and stainless.

MIG welders are a lot cleaner than Flux-core. Flux core has a lot of splatter and you have to remove a layer of slag on the weld - kind of like stick.