IMO, as both types of flanges are acceptable for the tank nozzle, the slip-o flange is used for smaller size nozzles, say less than 3", and the WN flange is for larger size.
The slip-on flange needs to have double welds per B31.3. The WN flange has only a single weld, but the pipe needs time to prepare a bevel for the butt-welding. And the butt-weld joint may have better mechanical strength compared with the fillet welds.