We're taking our 4th WDW trip in Sept this year too!
Teresa, how old are the girls and are they Harry potter fans? This will help us better assist you in planning.
We went to US/IOA on our trip last May. First thing I would do is consult park calendars for "most recommended" park days.
Undercover tourist and Orlando informer both have them for free. Touring plans also has them. That way you can see predicted crowd levels and chose what park looks best based on that. I think in general US/IOA is better to visit during the week. You might find doing a mix of WDW and Universal days to be the best scenario crowd wise. We found that one day at each Universal park was enough for us. We got the one park per day tickets. We rope dropped both parks and by 4-5pm felt we had done most of the highlights. We did NOT purchase Express passes (like FP at Disney but with a fee). Like at Disney if you get there early you can get a lot done. The Express Passes are free if you stay at one of Universal's deluxe resorts (1 night stay will get you 2 days worth of express passes) But we did it without and didn't find the lines to be that long except at midday. I personally wouldn't switch from WDW to Universal hotel just for the express passes. Like you said, too much of a hassle. I know a lot of people mention using Mears or Uber for traveling back and forth. It is about 30 mins from WDW. You might want park to park tickets (think Park Hoppers) that way you can hop between parks on the Hogwarts Express. Having Park to park tickets is the only way to ride it. We did not have hoppers so we missed that but will get on our next visit
Universal was really fun and a totally different experience than Disney. The Harry Potter sections have theming unlike anything at Disney. If your girls are HP fans they will want to spend a lot of time just looking around in those areas. Start following the Universal forum on the Disboards for tips/tricks. I learned a lot there!