TIG is always 100% argon. MIG can let you experiment with different gas mixtures depending on the wire that you are using and the quality of welds that you can accept. As for back purge gas on stainless, Nitrogen can be used instead of pure argon if you want cheaper option. It is a bit harder to purge all the air out due to it not being as heavy as argon, but it will work well once you displace all the O2.