It depends on what you want to do with it
"Better" really depends on your anticipated use. If you plan on doing any trailering to other locations, you want the centerboard. If you plan on going shallow areas, or areas that have unexpected shallow obstructions, you're better off with the centerboard. If you want to race (the O'Day 25 is probably not your best choice for that anyway), or have a real desire to point higher to the wind, have more control when sailing windward, etc, then you want the fixed keel. It's faster, more sail area, and points better. I have a 76 centerboard O'Day 25, and am very happy with it. We take it to the coast (400 miles away) 2-3 times a year, and even though it's something of a chore to unrig it, trailer it, and rerig it, it would be much more difficult with the fixed keel.
