The one that is most searched...? But it depends on what you mean by beneficial. In terms of traffic or English Spelling Kudos (ESK).
I tend to use the keyword suggestion tool here to help me better determine which spelling to go with. http://www.digitalpoint.com/tools/suggestion/?keywords=proxys&country=us http://www.digitalpoint.com/tools/suggestion/?keywords=proxies&country=us Maybe that will help you decide.
No doubt "Proxies". i recommend you to do some keyword research before targeting any keyword. You mus be select those keywords have high volume.