Someone just posted to this thread and it was a reminder that I should post an update on what we did and how the trip went.
In the end I did get Park to Park tickets, mainly so we could ride the Hogwarts Express. It was the right decision and everything worked out great. We only rode Hogwarts Express once in each direction (was down the third day we were there) and just walked between the parks when we wanted to swap. We did NOT get Disney park hopper and it was also the right decision.
We stayed off site at a rental by owner property and it worked perfectly. We were closer to Disney which was fine because 5 of 8 days were at Disney but even with the drive we were able to get to Universal pretty early (hard to motivate all 4 kids ages 9 - 17 to go to bed early and get up early) and really had minimal waits in either park.
Probably the biggest factor was picking the right days for each park. Low crowd predictions from Touring Plans was the number 1 criteria I used with park closing times (especially for Disney with the early Halloween event closings) being the second factor. (Yes, I'm a planning maniac. It's my job.)
We rode the Harry Potter rides multiple times with relatively minimal waits (one back to back trip at a time through the Dragon Challenge was all I could handle) and we even did multiple walk off/walk on trips through The Mummy (like 3 or 4 trips in a row). The Saturday had heavy crowds most everywhere so we used it for our second day at Magic Kingdom and with some strategic use of Fast Passes it wasn't bad. We also picked MK that day to see the fireworks because other nights MK closed early for the Halloween event. MK fireworks and laser light display that Saturday night was out of this world, well worth the crowds. Biggest negative was the crowds at Epcot because of the Food & Wine festival, especially the night we stayed for the Epcot fireworks (in the end we decided it was too long a day and the fireworks didn't live up to the wait and the Magic Kingdom fireworks we saw later in the trip were far better). The crowds didn't really impact ride waits (again, strategic Fast Pass deployment was critical) but walking around was painful at times.
Thanks again to everyone for your advice and recommendations!