Our boys were 2 and 4 our first trip there. They LOVED Universal, and we have been back every year since.
I like the two park idea. It's not like WDW, where it takes forever to get from one to another park. the parks at Universal are very close and walkable to eachother.
It's a tough choice, but I think I would go with the Studios if I had to pick one. The vibe is more laid back, the park is more spacious. do they still have the Animal Planet Live and the Barney Show? Those were huge hits, as well as the E.T. ride, Jimmy Neutron, and the huge water play areas. Just my two cents.