It depends on the size and type of your project and the location of the installation. You must consider:
Peripheral area for pipe welding and installation - for smaller, more compact spaces using field welding, electric fusion welding is better.
The size of the pipe. The fused system is only suitable for systems ranging from 20mm to 710mm, while butt welding can perform systems up to 2000mm.
Training, certification and experience will also affect your decision. The preparation of the pipe for the two fusion methods is a meticulous manual process, however the welding itself is the biggest difference. Electric fusion welding can be a more automated process using a welding machine with parameters that are suitably fused. Butt welding has more manual components in the process.