Good ol' stinky Decatur! I live in Springfield and just yesterday we had the windows open at work (Decatur's about 30 miles to the east of us) and said "It smells like Decatur!". When the wind blows just right, you can smell it! Just a way of life there and you get used to it.
My DH is from there originally lived there from the time he was 3 til he was in his early early 30's. He grew up in Mt. Zion, which is a decent area, and Maroa/Forsyth that others mentioned is nice as well. There are some lovely areas out by Lake Decatur - I'm not sure what you get for your money there, but definitely nice.
We aren't that familiar at all with the school districts, unfortunately (recently, anyway).
Our family lives on the far eastern edge of Springfield and are in Rochester School District. It is an EXCELLENT district. The area we are at is very close to the interstate (3-4 minutes) and is an easy drive to Decatur. It's not super close, but you can get there in 30-45 minutes depending on where in town you work. Not sure how you feel about that type of commute or not, but it is an easy drive w/ all interstate (my DH commutes an hour each way every day so for him it's not a big deal). Neighborhoods out here are wonderful, lots of kids, safe, and new/newer houses you can get between 150-200k. Lots of people around here commute from Decatur to Springfield, it's not a big deal. Just something to think about! Honestly, I would never want to actually live in Decatur (sorry!).....and my DH was not at all disappointed to move away from there.
Good luck with whatever you decide! Lots of big decisions. You have plenty of Central Illinoisans here so we're more than happy to answer questions.