I could wear black 100% of the time... but I like both dresses. Both would definitely show off your back muscles.
G-d, I hate that expression "cougar." But, I certainly don't want to look like one. I do wear flouncy skirts with camisoles, and a sweater. But, my camis cover everything and I wear a strapless bra underneath! I can wear shorter skirts, but never go more than a couple of inches above the knee.