What to Wear to the Ball?

Well wedding actually - my cousin's son in Richmond, Virginia at the beginning of September. The dress code is apparently "cocktail" whatever that means. I have made some discoveries, mainly that posh frocks are a) very hard to come by as no one wears/ sells them anymore. And b) they are astronomically expensive which means the cost of wear is ditto as i am unlikely to be invited to too many Buck House garden parties or the like. 

My judging panel (aka daughter and daughter in law) say either top left or bottom left/ top right. The latter (worn once so far) was the dress i wore to daughter Penny's wedding and i think that I will probably plump for that one. 

What do you think out there?