just my experience, we had AWESOME results with 2 years of preschool. my son was at that cutoff too - some said i could have sent him, some said no- either way i wasn't interested in rushing and as a rowdy little boy i made the decision to wait. didn't even call the school. i thought he was doing great in his preschool/daycare center the first year. he had a ball and did lots of fun crafts and worksheets, all about letters and numbers. learned to write his name, it was great. (he already knew his letters and numbers going in, so i didn't see a huge difference but i knew he was getting something out of it so i was really happy).
then we decided to do year 2- same class, same teacher, and OMG. now (the end of his S. year) he is spelling, adding numbers 1-10, and reading. not huge words, but he has picked up on phonics (and watching the electric company has helped too - this 5 year old knows about the silent "E" lol) i could NOT believe how much more he got out of it the S. time around. he sounds out words and one of our favorite car games is "how do you spell...?" and he will sound it out. he also reads beginner readers. not flawlessly, with a lot of help, but he probably reads half of the words. it's truly amazing.
granted, he probably would have learned similar things in kindergarten. but i was a girl and one of the oldest, so i am thinking with him, as a boy (maturing slower) i absolutely made the right decision for my son. he is a little calmer, listens better, and is a little less "squirrelly" in general. which was our goal. the rest was just gravy. :) good luck with whatever you decide!