Desirable can be described as something that people really want, it is suitable and is worthy to be desired.
"Mary's new hairstyle makes her even more desirable."
Desirous means to have the feeling of desire or excessively eager and wishing to obtain someone or something like money, could be through inordinate moves.
"A man of his calibre and integrity shouldn't be as desirous as he is. He is a public figure!"