I think I have it all. I think I will go for the SD slot (flash drive).
Macbook Pro Ports Not knowing much about flash drives, the measurements are 0.9 x 0.7 x 0.3 inches. Does that include that gray piece on the end? The port is bigger than an inch, but I assume that doesn't matter. What does matter is how much the gray piece on the end hangs over the port because I use the usb port on that side for my Logitech laptop speaker so I need room for that.
You've got your apples and oranges mixed up. SD slot and flash drive don't go together. It's either SD slot/SD card or USB port/flash drive. And the SD card slot isn't even next to the USB port, so I don't understand your concern in that respect.
Either an SD card or a flash drive will require a USB C adaptor to work with your next Mac, so they're equal in that respect other than the SD card adaptor being considerably bigger than the USB adaptor. (The SSD is USB C and will work without an adaptor.)
As for your concern about space, any listing you look at should include the dimensions of the device. I'll note that in my experience, I've never run across a device that didn't fit, but I've found that the tolerances are so small that a device that went in easily can be extremely difficult to get out.
