Have owned an O'Day 25 for 10 years and have sailed many times on Catalina 22s and 25s. In my opinion, the O'Day is a better built boat although they are older than most Catalinas and may show their age. There is an active O'Day community and a parts dealer who used to work for O'Day specializing in parts and information for and about them.
The 25 sails well and is fast compared to others in its class. The boat has a swinging centerboard --- not a weighted swing keel. There is a 1500 lb lead keel about 18" deep and the centerboard folds into it. When the board is down she draws 6 feet -- 3 feet when up.
The only problem I have ever experienced has been the centerboard becoming stuck in the up position - from barnacles or mud packed in the slot. It's easy to get down when the boat is hauled or by diving under the boat. Just don't crank it up as tight as it will go and this doesn't seem to happen.
Again, in my opinion, if the condition of the boats are comparable (and good) the O'Day is a better buy than the Catalina.