Hi Andrea,
Sumtotal was created in 2004 from a merger between Click2Learn (makers of Aspen LMS) and Docent (which should need no introduction), and Sumtotal then acquired Pathlore in 2005. So Sumtotal is made up of three of the biggest names in the Enterprise LMS world. Possibly the reason some Moodlers haven't heard of it is that Moodle predominantly (although certainly not uniquely) operates in the academic world, whereas Sumtotal (and the three LMSes which preceded it) predominately operates in the corporate/blue chip world. I've been working for a UK e-learning content development company for 10 years and have come across Pathlore and Aspen environments at our client sites many times, however not nearly as much as Docent, which is incredibly popular. Many Docent customers are now upgrading to the latest version of Sumtotal LMS, some major UK clients of ours are just migrating their environments at the moment. Sumtotal is massive, make no mistake about it.
Regarding the pros and cons of each, the big advantage of Moodle is that it is hugely configurable. Docent and those enterprise LMSes are all well and good if you want to deliver learning to hundreds or thousands of employees and create magnificent management reports and integrate with your HR system, etc, but it's a very formal, top-down approach to learning, and is nowhere near as collaborative and social (i.e. 'informal') an environment as Moodle. As 'informal learning' seems to be the big buzzword this year, Moodle supports that better than any proprietary system, so you could use the formal vs informal learning argument in your case too, try Googling that one! I have lots of informal learning links on del.icio.us at
http://del.icio.us/elearning2.0 too, more technical than pedagogic stuff, but may be of interest.
Moodle is also infinitely more extensible than Sumtotal, and I think that is its greatest advantage. The amount of out-of-the-box modules that come with Moodle and can be enabled/disabled is impressive, and the amount of third party modules that you can download and plug in it is truly amazing. It really is the most configurable and extenible LMS I've ever seen, and I've worked with most of the major corporate and academic LMSes in the UK market. In a nutshell, you will be able to make it fit the needs of your organisation's learning strategy, rather than having to do the opposite and design your learning strategy to meet the needs of a proprietary off-the-shelf package.
The other advantage of course is that it's open source. It is a hugely popular open source project, professionally developed and supported but with an immense user and developer community which means that informal support via the forums is excellent. As a popular open source project you can bet it will continue to be developed and extended at a much faster pace than any proprietary LMS. It is also so popular that numerous professional support services have sprung up around it (Moodle Partners) - so paid-for support is available if you want it. Check out
http://www.opensource.org/advocacy/case_for_business.php to find many more generic points and mythbusters about why open source is good for your business.
I don't think your positive outlook on Moodle would drop at all if you see any other proprietary LMS. In general I find the proprietary enterprise LMSes to be pretty badly designed from an administrative user's perspective, and overly complex for many organisation's needs, especially small and medium sized enterprises (SME's). If you are an SME, and wish to foster a social, collaborative approach to learning, Moodle is the way to go. Many of our clients, of all sizes including national public sector bodies, who don't have an LMS presently but wish to install one, are choosing Moodle. And the Open University has now gone live with Moodle for 12,000 students, as if you need proof that it suits the larger organisations too.
Good luck with your business case,
Mark