The weather, amount of time it takes for us to do each park and how tired we are, are our guiding influences.
IOA is not a great rainy day park -- Universal is better for rainy days, IMO.
IOA is a shorter park for us to get through. So, for instance, arrival day or a day where we need to get back to our hotel early generally ends up being a good IOA day (as long as it's not raining!).
Other things we think about:
Universal day = getting pretty toasted at Finnegans. This isn't a good day, say, for checking into a timeshare or hotel after downing three or four pints at lunch. Mythos & IOA is better for that
IOA is a park we get wet at, though. We're heading down this Saturday and we wanted a short park that we can enjoy from around 10 in the morning until we check into our timeshare at 4 (BMG at 8 that night - busy day!). I don't want to be soaking wet from the Popeye ride at IOA, having to sit in my car to drive over to the timeshare, plus having to check in with wet shorts. Can't do Universal that day because of the Finnegan's dilemma with pints; I seriously need to have my wits about me when checking in with the Westgate sharks. Hence we're doing MGM.
So weather, short/long park, degree of toastedness from tasty beverage consumption expected at either park, and the "wet short" factor are our biggest guides. Never mind tired
Have a blast!!!!