I agree with Paul.
I own Whois.Cart, ClientExec, and ModernBill.
Whois.Cart has potential, it's will soon be a very competitive piece of software... but it's not there yet. At the time (4 months ago) it lack automated billing functions specifically when a credit cards were declined. No noticafications went out to the customers.
ClientExec has great invoicing features which are very useful to web designer or hosting companies offering misc. products and services. You are able to create single invoices for projects to request payments from customers. However, ClientExec left our automatically processing through CRON Job. Because of this, everyday or monthly, you have to manually hit the generate invoices button in the admin and manually followup on any declines.
I wanted a solution that would rebill up to three time sending notifications if authorization failed. The last noticification would serve as a final attempt before the billing system would automatically suspend the account. Whois.Cart and ClientExec lacked this feature. I found it in ModernBill.
ModernBill practically automates everything from registering domains to creating and suspending hosting accounts to following up wth declines. This convenience isn't cheap. But in the long run will save you time and money.
With that said, I went with ModernBill and still loving it.