Gas for MIG: C25 vs straight CO2 vs argon, and bottle ownership vs lease

C25 (75% argon, 25% CO2) is the default for steel: smooth arc, less spatter. Straight CO2 is cheaper, penetrates deeper, spatters more, and is fine for thick dirty steel. 100% argon is for aluminum and TIG, not steel MIG; it gives a tall narrow bead with poor wetting.

On bottles: owning a bottle (buy once, swap-fill) wins for hobby use. A 125 cu ft owned bottle costs around $300 to $350 up front and $40 to $60 per fill. Leases make sense only if you burn a bottle a month. Ask the gas supplier about "customer-owned" cylinders; some will not fill off-brand tanks.