I would say WHMCS has the most users out there and a lot of people like it. We are using Blesta and it's good overall but it still lacks a lot of needed features. I would suggest getting a trial license for a couple of different programs to see which works better for your business.
Actually, for the cost perspective, hostbill is cheaper than whmcs. If you use WHMCS, you need additional module and cost which created by third party vendors. Third party vendor has probability to shut down and you will not get support anymore from them. If you use Hostbill, the module is created by them, not any vendors. But it's all depend on your situation and needs.