I would say, both WHMCS and HostBillApp have their own advantage.WHMCS + LiveZilla = Total Win.
Seriously, HostBill's livesupport is nothing compared to LiveZilla, which is free, and WHMCS is really solid, not only it IS solid, it also feels well scripted
I haven't experienced a single bug in HostBill, so you're kind of talking out of your --- there.WHCMS is the most known and from what I know it's more stable, bug free and more secure than any other.
If I would have to choose between the two, I would give it a go to WHCMS.
I have no idea if they have had bugs in the past as I started to recently use HostBill, but in the last two or three months I haven't had a single issue as far as bugs go.There have been bugs, but Hostbill is very fast at getting out the fix and adding it to the version that it effects. But Hostbill staff's attitude is disgusting.