a seed is better. because you can hide it much better. both in the electronic form and physical form
design a crossword
hide the words it as a grocery shopping list
a christmas wishlist
that seed is yours for life
..
wallet.dats can get corrupted. data loss, physical loss of usb memory stick, virus, electrical surges killing memory storage
and every time you use a new address or move funds to a new change address you usually end up needing to re-backup the wallet
But for the very paranoid there is also a good extra in wallet.dat = it cannot be generated with any seed, so only you own it.
With a seed, theres a chance someone somehow generates your same seed.. but with a wallet.dat, they would need to steal the file.
If you keep safe backups of your wallet.dat in various forms, I think it's a valid cold storage.