Last month I visited Universal for the second time. I absolutely loved it and would definitely go back again.
As we only has 6 full days in Orlando, we decided to do both parks in one day ($175). Due to series of unfortunate events which ended up working greatly in our favour, we were given complimentary express passes when we arrived a the parks. However, we still struggled to fit both parks into the one day. If you aren’t buying express passes and want to do both parks, then I recommend you do one per day. Single Rider lines are often much shorter particularly for rides that aren’t big rollercoasters as they extract loads of parents with children. The Harry Potter rides are generally the busiest so I recommend doing them once you arrive and then continuing on because it’s a loop anyway.
We got the Hogwarts Express between the two parks but in hindsight it was a bit of a waste of time since we had to queue. It’s not particularly magical, you’d be better off having more time walking around the parks.
I personally prefer Islands of Adventure but both are fab.