I think it depends so much on the school district and the quality of education, not necessarily if they go full day or half day. We're lucky enough to live in one of the top school districts in the state. My 5 year old goes to half day kindergarten. He is way, way ahead of where his friends are that go to full day kindergarten in other surrounding districts. He is reading better, he is studying more math, doing more science, plus they still fit in PE, Music, library, and art classes once a week. My friends that have kids in full day are all shocked at how much more my child is doing in half day than their kids are in full day.
In fact, we got a letter sent home in January from the kindergarten teachers saying that most of the kindergarteners have already mastered the entire curriculum for the year, so they're moving on to the first grade curriculum with the kids that are ready for it.
My advice, check out the full and half day programs carefully. Just because they go to full day kindergarten does NOT mean they will learn more than in half day. Quality, not quantity is what's important at this age, IMHO.