Why were Yisrael more likely to believe the second sign than the first?
Rashi: Because, when they hear that Moshe was smitten with leprosy for speaking Lashon ha'Ra about them, they would certainly believe him, since Hashem had stricken both Pharaoh and Avimelech with plagues for acting against 'Yisrael'.
Seforno: Because to cure leprosy that is snow-white (and is akin to death) is a greater miracle than the earlier one.
Why does it say 'if they will not believe'? It says (5:1) "va'Ya'amen ha'Am"!
Moshav Zekenim: This refers to the elders that Moshe gathered, but the others did not believe.