It is not better to export the private keys. Doing so may lead to funds losses.
To clear any doubts of new people which may occur after reading this statement:
Exporting the private keys itself does not lead to any loss.
The fact, that you only backed up a specific 'set of addresses' can lead to a loss if you receive funds to a newly generated address in the future (which you don't have a backup of yet).
So, as a backup.. use the seed / mnemonic code. Exporting private keys itself (for whatever reason) is not an issue.