There is a difference in drying curly hair with and without a diffuser.
If the hair is diffused, the air flow from the hairdryer (even tho less than a normal dryer flow) gets into the hair and roots more, therefore lifting the hair away while it is drying, thus leaving the hair airier and curlier. The steady air low does not disturb the curls too much unless the air low is too strong or you have fine hair involved.
If the hair is air-dried without a diffuser, the water in the hair, weighs down the hair while it is drying, therefore unless you remember from time to time during the drying process to squeeze the hair up, the hair will dry flatter but still defined.
Some of my recommended products that I like for curly hair are:
Prive – Curl Activator (medium hold)
Davines – Defining Cream (soft and light feel)
Tigi – Curls Rock, Curl Amplifier (crisper harder hold)
Ojon Hair – Animated Styling Cream (mild and soft)