Fusing.

Well it all depends on your criteria for "better". First, some fiber optic (FO) cables must be spliced and some must be fused, so you may not have a choice once a fiber has been selected. Which joining technique is correct for a fiber all depends on the composition and size of the glass itself. Usually a correctly fused connection will have a lower reflection and signal attenuation. However, an incorrectly fused connection cannot be repaired. It must be cut out and replaced. Most mechanical splices can be disconnected and reconnected many times without much difficulty. Most mechanical splices are relatively easy to do and any technician with the correct tools can do this. Fusing often requires careful polishing and a stable environment to be properly achieved. There are many fusible fibers that have excellent in-situ fuse repair kits that mitigate any assembly difficulty.

As I said in the beginning it all depends on the original fiber choice.

Both work well if done correctly, but which one is best depends on the type of fibre (there are many types), cost and the level of training of the installer (a poorly done fusion connection can be worse than a ok done splice connection).

Have you tried a Wikipedia search for the fibre type and splicing method?

Alternatively just buy fibre pre-fitted with connectors, it isn't like the old days when you had to carefully manually polish and fuse every fibre using an expensive tool.