(re-posting my response from the other duplicate thread that is now locked)
I always prefer getting Park to Park, not because of the Train (although it's a cool experience) but to have the ability to park hop. the parks are literally attached to one another so it takes minutes to go from one park to the other.
Will you have Express Passes (either free from staying at one of the deluxe on-site hotels, or purchasing them)? If not, I find Park to Park even more valuable. Crowds ebb and flow between the two parks and you'll find that sometimes the rides at one park have high wait times while the other park are low, and vice-versa. I love having the freedom to hop over to the other side if it's less busy there, or if the weather is bad, outdoor attractions (which IOA has more of) are shut down so I can hop back and forth to concentrate on the indoor rides.