Definitely WHMCS. While it does have a lot of features, the admin area is very easy to navigate. It's also highly customizable. I have it embedded in my site design and you can barely even notice that it's billing software.
As far as ClientExec goes, it is definitely simple and can get the job done. However a big draw back for me (not sure if this is still the case) is that you cannot credit a customers account. You basically have to manually create invoices with no charges or something along those lines.
I notice that most hosting companies chose to tell you that WHMCS is better. This is also right in my opinion, although it's not just rather it has the most features or the most popular. You have to think about every aspect other than your own.
When a client who is at another company using WHMCS comes to your hosting company expecting everything to be better and they see that you both use the exact same billing system as the one they switched, then they might have second thoughts.
I belive that a customer wants to feel secure and like they are with a one of a kind host rather than one that is just like every other hosting company out there.
Ableer Web Services, LLC
Website design - hosting - business cards - people who care. www.ableerweb.com
I don't think any billing software is going to be bug free, I've read bugs for both. I use Clientexec only because it comes with my reseller package for free. If I had to pay I would go with WHMCS because I like the way it looks, much more polished than CE.