I've seen both these terms listed in online music glossaries as meaning "dying away", but neither entry ever refers to the other. Are there any real-world differences in how these are interpreted?
FWIW, the go-to Dolmetsch page says
(Italian) fading away, expiring, dying away, spirando, en expirant
(Italian) failing, diminishing in strength, dying away, lacking
(Italian) extinguished, put out, gradually dying away to a whisper, calming down, subduing (Italian ) in music, similar terms include al niente (Italian), morendo (Italian), dämpfend (German), abschwächend (German), bis zum Nichts (German), en amortissant (French), hasta la nada (Spanish)
The basic answer is that composers use the word(s) they want to, and there's a ton of overlap in the multitude of semicommonly used descriptors.