Are there and advantages of using straight Argon vs Argon CO2 mix in mig welding? How about using just flux cvore wire and no shielding gas?

Different process for different material and different conditions. 100% argon I'd what we use for aluminum. Flux core wire design for no shield gas is best for outdoor windy conditions where your shielding gas would get blown away but when you want wire feed instead of stick to put more weld down fast.

I'm fare from an expert but each process and materials and conditions have their own requirements. Some corss over well others don't

The clear answer is ...it depends.
Factors are , What material are you welding ... stainless and aluminum for example need 100% argon. A lot of steels you can weld with mix. How thick is the stuff you want to weld ? On some thicker stuff I would not use flux core. On not so critical things I sometimes use flux core and no shield gas at all.
So it all depends on what you are trying to accomplish. As I weld a fair amount with TIG I always have 100% argon at hand.
Cost is another factor where mix comes out on top.
On the other hand ..... HOW DARE YOU TO GET BOTTLED CO2 AND RELEASE IT INTO THE AIR ... Do you want to kill us all ??

Scuba as lot of structural welding especially moment connections on structural steel gets done with strait flux core. So critical welds that all get x rated and thick material. It's nice because you don't have any starts and stops from having to change rods and you can flat put down some weld fast.

Dule shield is the easiest smoothest I have ever run but you need to be in a booth or shop for best results. I only ran it once but was able to certify unlimited with it running 2 1" plates in less than 30 min. Well 2.5 had a bad run in my wife on one set so ran about 10" off tossed the bad set of plates and started over. That solved the problem.

Your average home game welding machine does not have the horse power to run welds like that or even the wire thickness needed for that. Same goes for 6mm Multi rods we use for welding thick plates in the shipyard. Like welding the bow section of an ice breaker where ya need to fill some volume. After every rod you toss the grip into a bucket of water to cool it down enough to hold onto it for the next rod

Ok that gives me a good idea I have welded quite a bit and old enough learned with Acetylene flux and straight wire and brazed. Years ago got a stick welder. Never did aluminum and not much need to. Most welding is just repairs on different steel farm stuff a lot out side and we get alot of wind but mostly repairs. I do some small stuff at times in the shop and finally went to mig just seemed faster and also seems like it would work better on fine small accurate welds where its hard to strike an arc. Well got a spool of flux core and a spool of wire will get the bottle of argon and give it a test drive. Glad you mentioned the wind and makes sense it would blow the gas away and not work. Thanks

I always seem to be amazed at your knowledge and learn a lot from your post. I like I said I far from an expert. Just use what they said to on the job. Down side is my experience is limited. Most of it was always stick but the started bringing a lot more wire suit case stuff on the job about the time I changed jobs. Now it's all personal use so I'm having to learn more.

Almost bought a little Hobart 160 electric stick machine yesterday because it was small could be put in a shelf out of the way and was marked down to $400. But 20% duty cycle on 110 & 30% on 220. I wasn't sure if it was a good deal or if I should just wait and get a better machine. Didn't know how thin I could weld with it and if I could mod traps with it. Figured I needed a wire machine for the small stuff.

Straight Argon has lower thermal conductivity compared to 75/25 and the penetration characteristics will be different. Also it will often give a higher profile bead on mild steel... I always use straight aron on stainless and aluminum, Tig and Mig but thats a given... Straight argon is also sometimes used for certain tool steels. H13 and a few others.. For most situations and mig just plain ol' 75/25 is what you want

drop down to 40-50 amp amp range, 3/32 or even 1/16 rod and you can weld some pretty thin stuff.. I just built a flatbed almost entirely with a big idealarc machine. I could not get it in the shop because my power hammer and forge press were in the way so I ran leads out in the yard... It would have been a lot easier with my mig but ARC did just fine. Just had it her cranked way down.. Way back Id mod traps with 3/32 6013 on DC Negative.. I worked just fine, if a bit slow

Are you left handed or right handed, it makes a difference?

I use pure argon for tig welding aluminum and stainless and mig welding aluminum. For mig welding stainless I use a trimix of about 85% helium, 10% argon, and 5% CO2.