I'd go with the Spring Hill as well. The Marriott World is a nicer property but usually MUCH more costly. It's also much larger (you need a shuttle from the parking lot to the lobby for most rooms) and the property itself is huge. To me, this is a disadvantage with a 3 yr old who has already been exhausted by a day in Disney

The Marriott World pool is much better as well, but this will be lost on a 3 yr old. I think you have access to the 3 pools at the Little Lake Bryan complex (see if that is still true; used to be when I stayed at the Fairfield and Courtyard). Even if you don't, the pools there are nice. Smaller is better in your case. And, undoubtedly, you will visit Disney again...better to keep the 'cool' resort pool, glass elevators, etc. for when your child is older.

The Little Lake Bryan complex is nice and would be a better fit for your family, in my opinion.

The Spring Hill is just one traffic light away from the Marriott World, so the difference in distance to the parks is almost nil.

Just to point out, if you are not married to Marriott, you can often get the Disney value resorts (All Stars and Pop Century) for under $100/nt, making them very appealing and convenient in comparison to staying at a Marriott. They also offer free Magical Express bus service from MCO as well as free transportation throughout the property, so you don't need to rent a car at all if you don't want to....nice advantage!

Have fun!!