What is better plaster or liner?

Depends on what you're looking for. A plaster pool can be drained and painted but it does crack and certain chemicals can aid in the cracking process. A gunite pool would be a better but probably a more expensive choice than plaster. The liners are great. My brothers pool is an inground pool with a liner that was installed in 1983, in a flood and hurricane zone. It has never torn, wrinkled, or lifted up in any way. In a major flood zone such as we have here in south Louisiana, a plaster pool can lift up and begin to float. The liner is also very smooth and comforting to the feet when playing in the pool. My liner was installed only 5 years ago and we have been very satisfied, although it requires a different set of chemicals. A lined pooled should never be drained completely. Either way you choose, I would suggest going with the saltwater filtration system. It turns out to be less expensive in the end. Hope this gives you some things to consider.