We had a hard choice and (probably) made the wrong one.
My son had been in preschool and LOVED it (you couldn't get him to be quiet about all the things he was learning...he was so excited and THRILLED by all that he was doing) for 5 hours a day 4 days a week for over a year. (He had started preschool at 3 hours a day, 3 days a week, and we'd gradually increased the time.)
So we were fairly shocked that we had a choice between a TWO and a half hour "half" day (What??? That's barely enough time to drop them off and then you're turning right around and coming back again!) and an 8 hour full day. We wrung our hands and chose the full day and shelled out 3000.
He HATED it.
Now, he's super social so he loved recess and his friends and his teacher...but the whole experience can be summed up in a quote of his "It's long. And we don't do much."
He was bored out of his mind. He'd come home completely wrung out, exhausted, tears and tantrums. It would take him an hour to decompress, then it was off to soccer, and then racing to dinner/bath/bed. I felt like I never got to see him. I saw him at breakfast. I saw him in tears of exhaustion. I saw him at dinner and bedtime. I spent the vast majority of my time with him trying to "cheer him up", or refusing to fight, or holding him while he cried. Then the next morning he was bouncing off the walls excited to see his friends (and would be in the cutest huddle with the 3 of them outside of class), and then by the time school was over, he'd get off the bus or climb into the car with a big grin and wave to his friends and then collapse in tears as soon as we were out of sight.
He was hating "learning & school" by midwinter break. He was in a decent school, but his preschool had challenged him, been fun, been exciting. Kindergarten was learning colors and letters. He was bored (but we'd always kept that word out of our house), so he didn't exactly 'know' it. He just knew he was t.i.r.e.d. and unhappy. "It's long, mum. And we don't do much."
He had turned 180 degrees from how he HAD felt about school and learning in 4 months. By spring break we decided to let him finish out the year, but decided to pull him for homeschool for this past year. It took 6 months before he started to actually LIKE learning again. And it kills me, because as much as we do to keep him social...until after 4pm there aren't kids to play or to be with...and he's so charming and charismatic (he loves everyone and everyone loves him...don't ask me how, because I was the shy kid) he was Mr. Popular. While I realize that's not the most important thing in the world...it still kills me. Not as much as him hating school at FIVE, but a lot. (After 4 there STILL aren't many kids to play with, because parents are trying to -rightly- get their time in with their kids.
In any event, my advice based on our experience, would be this; If kindergarten would be challenging academically & socially...go for full day. If they already know their colors/numbers/letters even if it's ridiculously short...go for a half day. Don't let them start hating "school" in kindergarten. It's a really hard road to come back from.
Obviously I'm biased. But that was our experience.
Good Luck