In addition to what the others already mentioned, Liferay EE is supported for ~5 years after release, while you won't get upgrades for major CE versions once the next release is out - so you need to upgrade less often. Support includes patches for problems that you discover (on a service level) as well as cumulative fixpacks, typically more often than on CE.
For clustering - especially for bigger clusters - there are some additional clustering options in EE that optimize the cluster communication in order to scale better.
Liferay Sync can synchronize multiple document libraries on EE, compared to a single document library in CE
Liferay Developer Studio, the commercial extension of eclipse, works with more application servers - especially the "big iron" servers while the CE/Open Source version Liferay IDE works with the open source appservers/servletcontainers.
Plus, you're supporting further development of the product directly.
That being said, however you decide, please get involved in the Liferay community and help to make the product even better