The crowds shouldn't be too bad then but it will be busy. I would get there as early as possible and play it by ear. If the crowds start getting bad, you can purchase the Express Plus passes at any of the larger stores at the either park.
I wouldn't buy them beforehand...wait until you get there to see what the crowds are like. If you get there early, you can probably do most of the bigger attractions before the wait times start getting long.
There are two Express Plus passes you can buy. They have both the one park and the two park...the two park is good at either but you would have to decide which park to use the one park pass for.