I am using an AD8421 with a 100 ohm gain resistor. The datasheet for the AD8229 explains how to calculate the Johnson noise contributed by the gain resistor. I would also like to calculate the current noise contributed by the gain resistor, but I'm not sure how to do that. The underlying question is whether the extra cost of a metal foil would be justified on the basis of noise reduction, or whether a thin film resistor will provide comparable results. I need the maximum possible S:N ratio, but I don't want to use a $10 resistor unless it is clearly justified by lower noise.
To add to the conversation, metal foil resistors are useful for temperature stability, since they have low temp coefficients. If your circuit is intended for absolute accuracy over temp, then use metal foil. If you need good matching over temp, I would use matched resistors such as the LT5400. Otherwise, there is no need for the extra cost.