They are both completely different systems. I think WHMCS is Good .WHMCS an all-in-one client management, billing & support solution for online businesses.
WHMCS is one of the better systems I´ve ever tried, it gives to you a lot of good stuff to can manage your servers in one platform, there is also Clientexect, another good system.
It really depends on the persons needs. However, if you're a new startup in the hosting industry, I would highly recommend WHMCS to manage your clients.
I would have to agree with everyone else that said WHMCS. It is easy to setup and manage. Having everything built into one system can sometimes be a problem, but WHMCS works well.
WHMCS seems like a very good product, but I've never used it. If I had to pick between the 2 I'd choose WHMCS. I use Clientexec and it does what it needs to.