My son is getting dressed for half day kindergarten right now!
I like half day kindergarten. My daughter was in half day too. At our school it is only 2.5 hours. I don't think they rush through learning at all. They just don't spend as much time on lunch, snacks, rest periods, recess, bathroom breaks, and lining up to go places. Homeschool moms I know say they can do a days worth of school work in a few hours, it makes sense if you consider how much school time is not spent on learning. I think it makes for a nice transition from home/preschool to full days in 1st grade. I don't worry about the lack of recess because he has lots of unstructured time at home.
On the flip side, I wish they had time for art and music. I think it also makes for a heavier load for the teacher. Our teacher is responsible for 34 students because she has two half day classes. But there are only 16 students in my son's class, and there is an assistant there for part of the time. The student teacher ratio you described would worry me more than the length of the day, honestly. Kindergarteners need so much 1:1 time, and as you said, you do not want your son lost in the shuffle.
One more consideration, if it matters to you. My son is in afternoon kindergarten, and it drives me nuts. It really breaks up my day and limits what I can do with my two yo. No zoo, museum trips, extended shopping, even playgroups, library story time and just visiting friends who live farther away are all off our schedule this year.
I hope this helps. Good luck with your decision.