Id suggest hostbill, WHMCS is slow in adding requested adons, hostbill is VERY quick at adding requests and coding them.
WHMCS support is much slower than hostbill, WHMCS is a monoply supplier, hostbill isnt.
Both are secure & user friendly.
Hostbill is smaller so is less likley to be attacked by hackers.
Do the math, WHMCS or Hostbill? id go with hostbill.
Also if you wanted, you can get free hostbill licence on a vps.net VPS with ispmanager & a free SSL cert, your have a full billing server secured and backed up for 30$/month.
I find it funny that people can even have a opionion on hostbill when they havnt even tried it, handing out neg rep now (and positive to the people who actually reviewed BOTH)