Our friends who travelled with us last week stayed at the Hyatt thru Priceline getting it for $30/night. They got the upgrade for $40/night more. They thought it was well worth it.
We did the same thing when we stayed at the Hyatt 2 years ago. We saw the standard room and then saw the kidsuite. Night and day. My parents stayed in the old tower while we upgraded to the new tower. The problem with the old tower is that everything is of a darker decor, and is old, worn, and weary. It is still kept as clean as a hotel room would be. The newer tower has newer, brighter furnishings, which makes a huge difference.
If you plan on staying in your room a bit and have children in which you need larger space, then perhaps the kidsuites would be a better option. If you plan on being at Disney all day and just go back to your hotel room to sleep, then maybe not...
We upgraded, as did our friends, because the $40 upgrade cost + Priceline bid for the kidsuite is still such a good deal! That room is usually about $200!