Hi I.,
I've always found the best way to find a great preschool is to ask other parents whose parenting style is similar to yours. You also want to take your child's personality into account - for instance if your child is particularly rambunctious, you'll want a preschool that focuses on outdoor play, lots of "big" art projects, not too many rules, things like that. If your child can sit still and focus for a long time, or is a bit more introspective, maybe you want to look for a preschool that focuses more on academic type stuff. We looked at a bunch of schools for both kids, and schools that other people loved just didn't seem right for our kids - so it's really a gut decision on your part in the end.
What we did was wait until we felt our daughter was outgrowing her nanny, and then we put her in preschool full time. For my older daughter, that was when she turned 2. She really needed the companionship of other children at that point, and LOVED preschool. Our younger daughter started at 3, and for her we found we needed a school that was way more structured than what we'd found perfect for our older daughter. She had a much harder time with the transition, but after a year, she's really into it and loves it.
In both cases, we specifically looked for a preschool that had an "extended day" - in other words, they'd feed the kids lunch and put them down for nap, rather than being done for the day at 11:30am. I've found that preschools that only offer half-day are really not geared toward working parents, so it's best to find one that offers a daycare option. Once your child is ready to be "out in the big wide world" away from home, they can generally handle day care just fine. I suppose if your nanny wants fewer hours, then you could sign up for the half-day and have her pick up your son from preschool - but to me that seems overly complicated. Life's hard enough when you work full time for your child care to be too crazy!