S.K.
I think you would be happy in either place. We moved back to this area about a year ago and were contemplating Katy and Sugarland/Missouri City. We ended up buying in Sienna Plantation in Missouri City. We love it here. We decided on this area because it is closer to my husband's work. It feels like a community, not just a neighborhood. But I'm sure there are great places in Katy, also. My brother lives in Katy and is very happy.
If you work at Bellaire and 610 I think Sienna would be an easier commute because of the Fort Bend Toll Road. My husband takes the Ft. Bend toll road to work everyday and never hits traffic.
Btw, there is a Montessori school right in Sienna Plantation. My friend has a child there and is very happy with the school.
I hope this helps!