Does USB power delivery support arbitrary voltages?
Ok, actually reading the Wikipedia a second time answered my question:


The USB Power Delivery specification revision 3.0 defines a programmable power supply protocol that allows granular control over VBUS power in 20 mV steps to facilitate constant current or constant voltage charging. Revision 3.0 also adds extended configuration messages, fast role swap, and deprecates the BFSK protocol.


So, apparently the adjustable voltage power supply supports USB power delivery version 3.0 whereas the cheaper power supplies support only older versions of USB power delivery.

I think for future-proofing my purchase, I will select the adjustable voltage power supply. There may be some need for using the PSU with future devices that expect revision 3.0 support.