Honestly, they should all work about the same. PHPFox states the requirements and doesn't need much. Buddypress just requires WordPress. JomSocial just requires Joomla. All those are able to run on any sort of box. The performance issues are usually a result of the server itself and the settings you set up for Apache or Nginx. Honestly, I wouldn't worry about it at this point. Pick that one that is most comfortable for you to use and configure. If it does take off, you will probably have to make modifications, which will be easier if you are more comfortable with the code.
Once it does take off, a simple VPS should handle it no problem, but putting it on a cloud VPS will at least help you grow in the same environment by just allocating the resources you need.
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