Depends. Distant corrupted cities aren't much good for anything other than growing people -- preferably happy people to boost our score -- especially if they're on fresh water. In some cases it's better to get a temple as it allows for pop growth and allows the cities to start a cathedral, especially in the earlier stages of a domination victory when you expect to hold that city for quite a while.
On the other hand, building a Library is good when you need the border to expand quickly, or in this case, as you point out, simply because it's cheaper. In the final stages of a Domination victory you typically just want to get it over with as fast as possible. If the city is never going to grow beyond 6 anyway, a Library may be a better choice, assuming we have enough luxuries to keep a size 6 city happy, which ought not to be hard. At this stage of the game a Library's the right choice.
Note you can't build a cathedral until you've built a temple, nor a university until you've built a library, so really those are the only 2 reasonable choices.
On the other hand, building a Library is good when you need the border to expand quickly, or in this case, as you point out, simply because it's cheaper. In the final stages of a Domination victory you typically just want to get it over with as fast as possible. If the city is never going to grow beyond 6 anyway, a Library may be a better choice, assuming we have enough luxuries to keep a size 6 city happy, which ought not to be hard. At this stage of the game a Library's the right choice.
Note you can't build a cathedral until you've built a temple, nor a university until you've built a library, so really those are the only 2 reasonable choices.