Depends if you have early entry and if so which park has early entry. We just returned and did both parks in 1 day. IOA was the early park so we did the HP area, rode FP then the dragons a couple of times then took the train over to US. Hit both within the first 2 hours. You can always go back and spend more time in each area.
If you don't have early entry then I think which ever park does not have early entry is your best bet.