What is better, remove the old tin with the desoldering iron or with wicking braid?

Both methods are described in the IPC-7711/7721. For large surfaces (THT, plated through holes, large SMD pads such as cooling surfaces) it is better to do this with the desoldering iron, while for small surfaces (SMD Chip 0402 and even smaller, fine pitch components, BGAs, etc.) it is impossible to use the desoldering iron (solder extractor) without burning the PCB and therefore wicking braid is recommended. The advantage of wicking braid is that it creates a smoother (more planar) surface. This is especially crucial when soldering fine pitch SMD components.