I think that 309L is a good choice overall. It can tolerate a high level of dilution and resist cracking. Also, austenitic stainless will not reject as much hydrogen into the base metal and thus reduce the chance of underbead cracking also. I'm not exactly sure what affects it would have to melt some of the tungsten into the weld, but there are stainless alloys out there with tungsten already in them, so a little probably won't hurt too much.