The FTDI friend (a) will work, but the simpler USB/TTL adapter (b) will not work. As it says there, “This cable is not good for Arduino re-programming.”
I just received an A5 from Adafruit a couple of days ago; it came with version 1 firmware. If you want to install new firmware, be sure you read the instructions about changing the baud rate (in the Sanguino boards.txt file), since the v1 bootloader runs at a lower speed.
To replace the bootloader, I used a USB Tiny, which worked perfectly.