It's all down to the phase of the moon, which way the wind is blowing, etc. I've had great parlour experiences (Kitty in Decadent Divas for one) and ones where I would have had more fun wanking
and equally great times with indies (Seductive Scarlett for one) and ugly punts (Wet Susie).
And dammit sometimes a girl that gives a great performance one time can't be arsed the next (or even confirm or turn up).
Over all I'd say that parlour girls are more reliable in terms of actually being there at the booked time than indies but the service can be rushed or timed to the minute.
I book with indies because they can often be more personable and "knock on the door, you time's up" is very annoying if you just need another minute or two ;-)