\NewDocumentCommand is part of
xparse, which is 'LaTeX3-in-2e' code. The team have marked this part as 'stable', meaning that we will maintain this code for use with LaTeX2e in an ongoing sense. As such, there is no danger of
\NewDocumentCommand is intended for 'package authors' to define commands, while
\newcommand is rather more fluid as it is also used to create variables (macros used as storage). This makes
\newcommand a better choice if what you are defining is a variable: we do not have a 'document level' LaTeX3 command for this concept at present. So the decision will depend to some extent on how you see the macros you are defining (commands with no arguments and variables are pretty hard to distinguish!).
Predicting the future is risky, but I think it's extremely unlikely that any stand-alone LaTeX3 format would not define
\newcommand, even if the recommendation might be to use other methods. In any case, there are lots of other concepts that I suspect mean that for some arbitrary LaTeX2e document, you won't just be able to process with a stand-alone LaTeX3 format without adjustment.