I wonder if you need both?
You
could have both, Ian! What I would do is terminate the speaker cord in a 1/8"stereo plug. Then, I'd get an inline female jack and solder that to one end of a salvaged male USB-A cord, making a small USB-to-1/8"mini pigtail adapter.
Presto! Direct plug-in to your phone or any 1/8" jack, and the ability to plug in via USB to your PC if needed.
If you do USB, observe the usual precautions about which lines supply power and which are for data, as shown in the link you provided.
All the best,
Dan. (...who can almost smell the solder melting now)