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.
You have it exactly backwards .... Nearly all large diameter flanges on API 650/620 tanks are slip-on or similar. Smaller diameter flanges are sometimes weld necks.
You are correct as weld neck flanges are stronger than slip-ons. But they are significantly more expensive. This is an important cost factor in the competitive bidding market for storage tanks.
Most importantly, IMHO, there is no reason to specify the robust and expensive weld necks in very low pressure service such a storage tanks...
