They're both nice dresses but the black one doesn't feel very partyish to me. The black one is very safe.
If you were going out for a lovely lunch during the day, then absolutely the black one would be perfect, but if it's a fancy evening party then I think the red would be nicer. It's a fab colour and a lot more fun I think.
It's your 40th and it's a party! If you can't wear a short red dress for for your 40th birthday party, when can you? I also disagree with the PP who said it looked cheap - when you click on the picture of it being worn by the model, it doesn't look cheap at all and you can tell when you zoom in that it's a quality fabric. Yes, it's short, but the long sleeves and the high neckline give it some class and the detailing is actually quite high-fashion!
To be honest, if you're 5'7" and a size 10 you'll look fab in anything but I'd go with the red.
(PS I personally find Coast very boring, safe and a tiny bit frumpy!)