i think the consensus is argon/co2 mix.
i did use straight CO2 and was okay for mild steel, but changed over to Argoshield light, as i'm doing more with stainless and the finish is so much better.
i'm using BOC as my supplier, granted £70 a year or so and £40ish for a refill, it is expensive, but cheaper than paying someone to make all the bits i've made with my mig.