It's better to buy it once you're in the parks and certain you need it. (If staying onsite isn't a possibility). We didn't need it our last trip but it was still nice to have for shorter walks in line. We did save a small amount of time using it towards the end of our trip. That was nice with our youngest not liking the same rides at the other two kids, but not a huge deal - just a nice perk.
Is it just you and your wife going? If so, don't forget about single rider line. Before we had kids, we relied heavily on single rider line. (We used to go over Christmas break). Not every ride has single rider lines, but it still helps quite a bit.