If you ONLY want to proxy check and do nothing else, scrapebox is powerful, but may not be the best choice. Its built to test proxies and all and has powerful features, but proxy testing isn't its focus. I mean like with GSA the entire proxy checker program is only about checking proxies so it may have more features in that department.
That said, I use scrapebox to test my proxies and always have. However I use only private, shared and back connect proxies, all of which are paid. So I spot test them but thats about it.
I have done extensive testing of proxies in the past for scraping proxy lists and testing etc.. but I never got into port scanning etc...
@proxygo would probably be a better person to ask, because he specializes in proxies and would know.
He also deals with public proxies and for me its a case of production so even if I needed public proxies I wouldn't, at this point in time, setup any software to go harvesting and testing them, Id just buy them from someone like proxygo because for the small cost its not worth my time.
For you though, it may be worth it.