If I have to pick I would say mornings because they let you know were they are. I like hearing all the birds gobble, thats what makes you want to go! That being said, I'm like Adrian, mornings and afternoons are good. I've found if you get one to answer in the afternoon the odds are better he'll not have hens and come in.
+1 on everything mentioned. I usually have more fun in the mornings, but it generally seems easier to kill them in the afternoon.