I've decided to buy a hardware wallet and have been comparing the Ledger Nano to the Trezor and the "Nano S." It appears that in order to access your bitcoin, the Trezor and the Nano S only require that you have possession of their device and a four-digit PIN. I like that the Ledger Nano has the additional requirement that you have possession of their "security card" or their smartphone app which has been paired to your device using the security card.
I understand that the Ledger "security card" is theoretically vulnerable to keylogging malware when dozens of send transactions are made from one infected computer. Does their smartphone app address this vulnerability well? Is the app reliable on Android phones?
You can still buy a Ledger Nano (original design) that required the security card. The new Nano S does not and is a better wallet in all respects, also works with Mycelium on Android. The Nano S is twice the price of the Nano about $64 and worth it.