Since you asked... I did a simple comparison test today, and lithium is much more hygroscopic than ammonium. Not scientific, mind you, but a small pile of each produces a wet spot with the lithium in fairly quick order (in about 50-60% RH), but the ammonium seems to be fairly dry and unclumped still.
In fact, I wonder is the ammonium is hygroscopic any more than the normal sodium is. They may be about the same.