I think that having each card alittle different is the way to go.
I just did a set of cards as a TY to my SIL mom who bought our bridesmaid dresses for the wedding.
I used Petal Prints and Lexicon of Love
I had monocramatic colour combinations:
Barely Banana/Apricot Appeal flower with stem
Bashful Blue/Night of Navy leaves
Pale Plum/Perfect Plum flower that looks like delphinium (sp?)
Certainly Celery/Always Artichoke leaves
Pretty in Pink/Regal Rose large daisy
Then because there was 2 in each colour I would make sure that they each had a different sentiment from Lexicon of love.
So they ended up all being very similar but different
have fun whatever you choose