Advantages of Trezor:

• Simpler to use. All coins/tokens can be accessed in just one page.
• You can see all the address on the device screen before confirming.
• Has a password manager which is really good.

Disadvantages of Trezor:

• Has lesser coin/token support than Ledger Nano S
• Has only one computer inside. Easier to hack.

Advantages of Ledger Nano S:

• Has more coin/token support than Trezor.
• Has 2 computers inside, which makes hacking it harder.
• Has a casing that you can fold to keep it's screen safe from scratches.

Disadvantages of Ledger Nano S:

• Complex to use. You have to install each coin/token app to the device first before you can use it.
• Due to the limited screen size; when transacting, you have to wait for the address to scroll across the screen just to check the recipient address before you confirm.
• Though it has more coin/token support. You can only use limited coin/token at a time because the memory of the device is limited.

Conclusion:
Which one is better is subjective to your needs. I need features that can only be found exclusively on each device so I have chosen to use both. I use the same mnemonic seed for both so I can access the same wallets on both devices which make my life easier too.
I suggest you use the Trezor because of the handy password manager and you can access all coins right away.
I suggest you use the Ledger Nano S for long term investment coins, since it's a hassle to install/uninstall apps just to access every coin/token you want. And, it's not so friendly when transacting all the time because of the small screen size.