Have you already checked out The Parents Guide to Preschool? It includes information on the academic, social and developmental elements of preschool. http://www.education.com/grade/preschool/
 
Also, here's an Education.com article debating the merits of academic preschool vs. play-based preschool (http://www.education.com/magazine/article/Ed_Academic_Preschools/), and one about phonics per developmental stage, from infant to school-age (http://www.education.com/reference/article/Ref_Phonics_Whole/)
 
Good luck with your decision! If you're looking for insight into the choices other parents made and what worked for them:
 
Our daughter went to a play-based co-op preschool, and she/we really loved it. The school had various play stations (art, dress up, music, climbing structure, etc.) inside and outside that volunteer parents and paid teachers would rotate around throughout the four-hour session. As parents, we'd observe the children at play but wouldn't interfere except for safety measures, answering questions the children had, or helping to mitigate or resolve conflict. There was also circle time where the teachers would read to the children or teach songs, and children always had the option of reading books (with or without adult help) on their own time at their own pace, or they could practice writing or painting, or let their imagination take them wherever it would with the materials before them. Every day at the preschool provided learning (for the children and the volunteers), and once-a-month the parents and teachers would gather (sometimes with guest experts) to discuss academic, social or developmental topics that supported or improved our parenting and helped us bond and thrive as a community. My daughter is in first grade now and has been doing very well in school both academically and socially.