Modernbill - If you are considering modernbill, I would personally go with the v4 or v5 and not Plesk. I seen the Plesk demo and it seems to be a stripped down version of Modernbill. I'm not a fan of the interface. We used Modernbill 4 but started eying WHMCS.

WHMCS - Besides the fact that almost any hosting company you run into will use this, it is pretty darn good to use and feature packed. The automation is superb, the license is affordable, and the support is always there to assist you. However, there are a few quirks, mainly of which I notice is the difficulty implementing a few marketing schemes. Example, if you wanted to offer a few months free on the hosting, this seems to be something you have to do manually and not on order checkout. Overall, I would prefer this over the others. I haven't tried Clientexec but looking at the demo interface, it just doesn't match up to WHMCS. One ultimate plus for WHMCS is that depending on the registrar you use, the nameservers and domain contact information can be edited right from the client area using API.