We're just back from 7 nights WDW followed by 2 nights RPR. It was our 1st Universal trip, so I'm certainly no expert on this. But, my peak season observations were that FOTL was not really an advantage compared to WDW's FP system. The reason is that during the busy season, everyone at the Universal resorts, plus everyone buying the Express Pass is getting the same "FOTL" access - - compared to WDW's metering system w/ the FPs, where only so many FPs are given out per day for a ride, and times are return times are scheduled to limit the number of returning riders. So while at WDW we waited in line maybe 10 minutes max using our FPs, at Universal it was 15 to 30. On 4/6, we waited 35 minutes for Jimmy Neutron, w/ all the other FOTL folks, and I kept reminiscing about WDW and the fewer FPs, but shorter waits. WDW took more planning, but it felt like I was in control. At WDW, I knew we had to be in the parks before opening each day to get the FPs we wanted. At US/IoA, I mistakenly thought FOTL meant we could take our time - Mistake! If it's a busy season, get there before opening and get to the attractions w/ no FOTL (like Pterodactyl Flyers -something like that name) ; and then hit the rides you most want to do.
During a less busy time, I imagine US/IoA could be paradise w/ the FOTL, but at peak season, I was wishing we had done US first, Disney last. WDW was more organized, better staffed, better controlled. We liked US/IoA very much, but concluded after this trip that Disney is a VERY tough act to follow.
Happy planning!