i agree with welding1, that 309 is the filler of choice for most "stainless" to carbon steel welds. he also mentions that dilution can cause a problem.
my experience has been that if i am to machine this connection after welding, i will use er347.
i am not a metallurgist, but i suspect the niobium and tantalum prevent the formation of "hard spots". all other properties are similar to 309--it's just a little pricey.
finally, as diamondjim mentions, if this is a liabilty issue, let the DE's come up with a welding procedure. maybe they'll bolt it instead!