As far as I know, in Bitcoin Core is impossible to show the seed of the wallet for security reasons, while in Electrum is very easy to show the seed and it is shown even in the wizard when creating a wallet. Maybe there is a way to show it with Bitcoin Core on the command line, but I hope not, since it's pretty stupid, I think it just adds an huge exploit possibility. A hacker gets your seed and he gets all future addresses you create... not fun. I would never feel safe that way. Also seed derivation was possible with Electrum if you had a master public key and some other keys I think... im just too paranoid to even bother and I stick to the basics.
Im going to get rid of Electrum as soon as I install 0.16. If I can't import the keys I guess I will need to send the coins to myself.
@cellard The core does not provide the seed of the wallet, but the master private key, and this master key is hidden and encrypted in wallet.dat, to have access to it you must export all the keys of the wallet at once giving the command in the console:
the master key will be at the beginning of the document.