I would personally choose WHMCS for two reasons. Firstly, it's familiar with both web hosts and their clients, this helps. Secondly, there's a huge number of useful plugins available for it. It really is the industry standard nowadays.
If you have to use either one of them go with WHMCS, Hostbill is overpriced, the support is next to nothing and if you need them you have to pay for a ticket. but if you use WHMCS keep it secure as much as possible with Mod_Security / Imunify360 so you don't have to stay up all night incase there's a patch. But yeah WHMCS over Hostbill.
I would suggest go with WHMCS. You will get lots of option to manage various things related to billing as well as other things too. It is easy to install and manage. We use it from too many years without any hassle.
The automate billing reduces your time. It is very user friendly and secure way of billing.