I'm partnered with RPM and have found them to be a good supportive network so far, however both networks have there pro's/con's. I take it Fullscreen have offered you a 70/30 split and RPM a 60/40? So there's 1 bonus of going with fullscreen. Not sure what Fullscreens music library is like but RPM's is extensive. Which network typically replied to you quicker with emails? As when you get partnered you will be relying on quick emails to get things sorted fast etc.
There's just a few things to consider but like others have said just take a look around and make your choice, the good thing is that both are 1 year deals ( I think).