We have visited in both Sept and May. Crowds were generally about the same. Lighter than usual but still a fair amount of people. Wait times 5-20 minutes depending on popularity of ride. Weather was much better for us in May. Warm and sunny and a bit humid but not like the heat and 90% humidity of Sept. It also tends to have a lot more afternoon thundershowers in Sept.
We have done "just Disney trips" and now have done 2 trips where we split time between Disney/Universal/
Legoland. We have found that 2 days at Universal theme parks is enough for our family to see and ride everything we are interested in. We usually arrive right at open and stay until 3-4 pm. We do just one park per day. Universal definitely has an older crowd (not as many kids) and the atmosphere is generally louder (rides & theme music alike). Rides to us are more thrilling there. Themeing is very comparable to Disney IMO perhaps even better in many areas (Harry Potter areas, Kong) We drove by Volcano Bay and the big Volcano waterslide is looking pretty cool. I guess its supposed to open in June so you may get lucky with a "soft opening" but I haven't followed if construction is still on track.