Both now have a user interface in several languages.
Personally I prefer the Trezor, for the following reasons:
On the Trezor it is very easy to use different passwords to cut your wallet into multiple smaller wallets.
So much extra trouble for hackers.
Possibility of different wallets for several people of the same family for example.
Possibility of wallet perso vs company.
The Trezor makes it easy to check the addresses on its screen. The screen of the Nano S is really too small.
The Trezor allows you to see all its reception addresses, not just a new one each time you want to receive corners.
The Trezor allows to better see what has been received from who and what is gone to who thanks to its labeling stored encrypted in Dropbox.
The Ledger Nano S makes it possible to use a password, but with difficulty and if one binds it to a special code PIN, one decreases the security.
The Trezor and Ledger both support a limited number of cryptocurrencies. An important difference is that with the ledger it is necessary to constantly uninstall and reinstall applications if you need to support more than 3 currencies at once. With the Trezor, all supported currencies can be used simultaneously without installing / uninstalling apps.
The Trezor is used to export transactions to Excel. The Ledger does not allow it.
The only advantages of the Nano S:
Supports some extra corners.
A bit easier to enter his PIN code.