What benifit will a hardware wallet like the Ledger Nano S give me over just having a thumb-drive with Tails?
They have better physical security. That is, it'll be more difficult for someone with access to the device to attach a hardware keylogger, etc. (Though the hardware wallet manufacturers overstate their hardware security, I think.)
Although very difficult, it's still slightly possible for someone to attack your offline computer via software. For example, even air-gapped computers can be attacked by exploiting the USB flash drives used to transfer data to/from such computers. An attacker could modify any scripts/programs on the flash drive to attack the air-gapped computer, or could theoretically even exploit a bug in the USB-flash-drive kernel driver on the air-gapped computer. Etc. Dedicated hardware wallets probably have less surface-area for such attacks. (And if you plan on using Tails on an Internet-connected computer, then its exposure to software attacks is significant.)
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