If you are on the Mac, load it as an AU ... there should be little difference in the functionality of the two, but if you have the choice, the AU version will rely on built-in functions of the system more than the VST version, and will probably run faster with less of a RAM hit.
You can always try both and watch the Activity monitor to see which version is running heavier on your machine ... see this throead for mor info:
[url]http://www.macprovideo.com/forum/audio/propellerhead-reason&id=1269[/url]
