There isn't any difference in strength between the flat and coil spring actions.  I like the "feel" of the flat-spring better, and think they're easier to take apart, but that's just personal preference.

Supposedly, the strongest actions were the later blued "express" actions, those without the paneling.  Next were the blued paneled actions.  Then the casehardened paneled actions.  I don't know whether the casehardened express action is stronger than the blued paneled action or not; I would think so, just from the added thickness of metal on the receiver sides.  I think C. Sharps makes both the express and the paneled actions, and the steel should be stronger than the originals.  However, a .30-40 Krag, unless you have plans to stoke it up to .30-06 or higher pressures, should be fine in a blued paneled original action, as they were made that way by Winchester.  It might be easier and less costly in the long run to get one from C. Sharps, unless you have a line on an old action already.