USB downloader is a USB-to-UART converter. It can be used to download firmware form PC to Microprocessor devices.
TXD led, RXD led, Power led and 6pin @ 2.54mm bus sockets.
There are two reserved pins(RTS, DTR) on M5Stack USB Downloader for other applications.
I don't get anything wy, so I don't have any opinion.
USB-TTL UART Serial Adapter (CP2104)
Doesn't work with MacBook Pro directly, found only one way how to utilise it - using 7in1 USB HUB, not mentioned what type of drivers needed - you can find it from Silicon Labs web page, search for CP210x USB to UART Bridge VCP Drivers