I think either of those would be clear enough - not sure which is the better Italian. You
need both, of course, since Adagio is a tempo (mostly) and rubato a nuance of performance.
One person's adagio may be a lot faster or slower than another's, but that's all good,
because some people will be able to make it work at tempi quite a long way from your
original idea and so putting a metronome mark is not really as conclusive as it seems. If
you do use a metronome mark, use the Adagio as well; it gives more clues. You might want
to use poco rubato or molto rubato instead of rubato on its own, depending again on your
idea of how it should go.
On the other hand, it's quite interesting to leave it
wide open and see how people react to it. Some music seems to end up the same sort of
tempo and style whoever plays it, other music attracts extremes of interpretation. Yu
might find out stuff about your music you didn't know by letting performers work out
what's right for them.