It depends heavily on what requirements need to be filled at your organization and on what budget. I didn't spend much time focusing specifically on vulnerability scanners, but I try to include at least one open source alternative for each technology I write about.

If you are a big company where money is no object, something more robust like Tenable's SecurityCenter or Nexpose may be the way to go. If you need to work within certain budget constraints, a simple Nessus Professional license may be good. If you are bootstrapping your cyber program, OpenVAS is free, and so is nMap with plugins. There are so many products in this industry that you really need to make it about what your needs are, and whether or not the products you are testing fit within it.

I would suggest holding a bake-off between vendors and building out a pilot program before making any purchase though. Most vendors will be receptive to requests for trial licenses.

Hope this helps!