The only real difference between the two is that a custom prebuilt PC will allow you to tweak the specs and configuration to your specific liking, such as a different case, cooler, and various other ...
Custom-built PCs often have higher-end ... which would see more spending on ancillary PC parts such as a PSU/Case/Cooler. A Quality PSU: Power supplies (PSUs) are often the most easily overlooked ...
The HP Omen 45L isn't just my favorite gaming PC, but I've even built a complete custom PC in its case myself, and that's not something I'd even attempt in most prebuilt gaming PC chassis.