you need stainless wire and argon to weld stainless, and think you can join to mild with the stainless wire, but cant weld stainless with steel wire and co2, not 100% sure but someone will let you know for sure.
Lots of people always pipe up about using stanless wire, or even better 309 wire.... depends what its for. I think this is a perfect example of something it really doesn't matter on.