The "old" kid in preschool--ok or not worth it?

I've been toying with doing a one-day preschool next fall, but the age cut-off for most seems to be Aug 31. So my little guy would be in the 2 year olds class, even though he'll turn 3 on Sept 24. I'm wondering if it's actually a hindrance rather than a benefit to have him in school with mostly younger kids at this age--especially considering I really want him to be potty-trained sooner than later! What do you think? Anyone doing preschool next fall?