Thank you for your inquiry. The difference between JomSocial Standard and JomSocial Pro is that with JomSocial Standard, you will have the 'Powered by JomSocial' link-back in your main page while this link-back is not available in Pro. Most of our customers prefer Pro to Standard due to the fact that they do not want this link-back to be shown in their page.
Hope this answers your question. Let me know if there's anything more that I can help you with.