I agree that Cpanel/WHM are the most desirable control panels. I have used Plesk before only on the admin side and that was just to help someone out who had a problem and I found it to be very limiting in what it can do compaired to WHM. I have used the Ensim from a user before and also found it very basic and somewhat limiting compaired to Cpanel.
You made mention that you where looking at getting a SB server. Take a look here.
http://www.serverbeach.com/catalog/products.php and please if you have time try some of the control panel demos. This way you can see what features are in each product. Make sure the automation product works for the control panel you want. Or make sure the control panel works with the automation software you want.
I know alot of people whom use WHM Autopiliot in conjunction with Cpanel/WHM. While instant account setups are best for the client since they get access to the server right away its also bad for the company. Just as was mentioned above as spammers find out about your instant setup policies they will flock to your accounts and signup with stolen CC info or stolen paypal account info. Then before you know it they will have an account and will be spamming the world on your servers. As im sure your aware being the owner of the server your ultimatly responseible for the data that is contained and is transmitted from your server.
Most clients I have found to be understanding of a breif waiting period from signing up till account activation when we explain to them the reason for the delay in verifying the payment information and contact information is correct.