I'd characterise Beethoven as prioritising harmony first, rhythm second, and melody, if at all, last. So with Beethoven, melody is an emergent property rather than anything else.
Chopin on the other hand wrote melodies and then harmonies to accompany them. Typically, I'd also say that his pieces have a lot less drive, as his use of rhythm, especially syncopation, is more, say, conventional.
You can find an easy example of this in Beethoven's first sonata, at the tail end of the first theme(? term correct?), where the suspension is written as a single key on the 1, and the remaining chord is added on the two. The normal way of doing this would be to have the suspension be a full chord with one stray key, then resolve that one key.
As an aside, if you play either, you'll find that Chopin has tried to make his pieces playable, e.g. used scales where the longer middle fingers can rest on black keys, with thumb and pinky on white keys, whereas Beethoven put his vision of harmonic relations first.