Objectively, heart rate, breathing volume and rate, posture, muscle tonus (relaxation), and I would expect but haven't observed directly changes in blood pressure, brain waves, etc.
Subjectively, people report a variety of sensations, warmth or coolness, tingling, a fullness or emptiness or both, etc.
edit: I forgot to mention accelerated healing (the whole point of Reiki, der! lol)
Well, there are lots of different types of meditation so it's hard to give a brief, comprehensive breakdown. FWIW, the "destinations" of both meditation and Reiki are congruent, if not identical.
Metaphorically, you might say that meditation is like a lamp while Reiki is like a laser. (It's not a very good metaphor.)
The main specific difference is that Reiki is for healing. It engenders and accelerates healing physically, emotionally, and mentally.
As far as the subjective sensations of chi flow they tend to be more dramatic with Reiki than meditation, but again, so many variables that it's hard to give solid generalizations.
Subjectively, people report a variety of sensations, warmth or coolness, tingling, a fullness or emptiness or both, etc.
edit: I forgot to mention accelerated healing (the whole point of Reiki, der! lol)