Pre pre school?

We started Darby and Gabe in daycare/preschool one day a week last October.  They are 4 1/2 and 3 1/4, respectively.  Their "school" is 20 miles away, in the opposite direction from work and I don't love it there.  We just found out one of our best friends will be running an accredited Pre-K/preschool program this Fall-two blocks from my house!!!  Darby needs to prepare for Kindergarten, Gabe is a wild tornado of a boy and we need care at least two days a week.  I know we need to transition to a more structured schedule, since I am quite laissez-faire, and my stress would diminish greatly if I didn't have to rely so heavily on our moms, but really, I think I just feel like the rug is being pulled out from under my idyllic, relaxed life with my babies.  I know it would be good and healthy for all of us, (I'm not the most patient of people,) but I thought I would have a couple more years. I feel like I have the perfect setup for right now, working only three days a week and being a stay-at-home mommy the rest of the week.  How much "pre" do our kids really need for school?  Am I finding excuses to assuage my mommy guilt?  Am I holding them back for my own selfish reasons?  How do we know when we're justifying and when we're actually doing them justice?