K.W.
Whoa, we have so much in common. We also live in Parkville, attend St. Therese, and have three kids, ages 11, 8 and 2. We also looked into both options, and decided to go with public school. Although our kids attend English Landing instead of Union Chapel, pretty much all of the schools in the Parkhill district are excellent. They consistently rank among the highest in the state as far as test scores go. Of course, we have friends that attend St. Therese, and by all accounts, that school is wonderful as well.
A couple of things that affected our decision: while St. Therese is an excellent school for the average kid, we were concerned that they wouldn't have the resources if our kids needed special ed, either on the gifted end, or with a learning disability. Although we didn't know it at the time we made our decision, we wound up with kids having both special needs, and Parkhill has been able to accommodate them wonderfully.
Also, we planned to stay here all the way through our kids' educational lives, and although St. Therese has a great reputation, there are not many Catholic high school choices north of the river. The only one is St. Pius, and I think its reputation is just okay. My husband attended St. Pius when he was in high school, and he didn't really want our daughters going there. Because we didn't want them starting high school not knowing anyone, we ultimately decided on public school. I can honestly say we have never regretted our decision.
Also, if you haven't bought a house yet, have you considered Riss Lake? The Riss Lake kids go to English Landing, which is the highest-ranked school of the Parkhill district. Also, Riss Lake has great amenities: three pools, a fantastic swim team, tennis courts, a playground area, and a really great group of neighbors. You'll have so many friends right away. Please look here before you make your decision.