It's quite simple.

A seed will recover ALL addresses from a specific wallet. Used or not. So most of those addresses you'll probably never use.

If you go the private keys route you'll only backup keys that are actually being used and known to the blockchain. Advantage is that you can import a key at any time to any wallet. Also, if you add an address to a wallet for which you have a seed but do not create a new backup, whenever you restore that backup the newly added address wont be restored and thus will be lost forever.

I prefer keys over seeds but that's just me.