I'm about to sell my licenses for both. Here's why: you have to have or scrape huge lists for GSA SER to run fast. Google is banning IPs faster than ever before, meaning you're stuck scraping other engines that aren't as good and have pages that may not be indexed. I personally can't handle spending time on software, its insanely boring.
GSA has a HUGE learning curve also. I'd rather buy blasts somewhere like BlackHatLinks and be done with it. More expensive in the long run in terms of money, but not when you consider how much time it takes to be good at using SER. If you don't believe me look at the GSA forum and how many people struggle with using it.