Q does explain the reason for his decision here, in the script. I've emphasised the important aspects:
TROI: They made you human as part of your punishment?
Q: No, it was my request. I could have chosen to exist as a Markoffian sea lizard or a Belzoidian flea. Anything I wished as long as it was mortal. And since I only had a fraction of a second to mull and I chose this and asked them to bring me here.
Q: Because in all the universe you're the closest thing I have to a friend, Jean-Luc.
Let's break this down a bit:
Mortality: Q couldn't choose any species he wanted; it had to be a mortal one. It's not entirely clear whether Changelings are immortal or not, but it does stand to reason that shape-shifting beings made of a morphogenic enzyme could be immortal. Of all the possible species Q could have chosen that are mentioned in the question (the Prophets, Organians and Metrons), we actually don't know whether they even are mortal; surely species as powerful as that could well be immortal! So, by placing a restriction on mortality, that too restricts the species' power; a species that is very powerful may have overcome their mortality.
Time: Q didn't have long to think about it, so he couldn't necessarily have a long, well-thought out choice made.
Friendship: I think that the 'friendship' excuse is quite important. If you had enemies what would you prefer; using your sole intellect, or having others help you - he does say that Picard was the closest thing he had to a friend. By becoming a human, it means he can more easily assimilate (no pun intended) into the predominately human community of the Enterprise-D.
And whoever said humans weren't powerful? The basis of the episode 'Hide and Q' is that humans, although somewhat limited, will soon overcome these limitations. Q himself says:
Q: Well if you'll stop interrupting me. This is hardly a time to be
teaching you the true nature of the universe. However, at Farpoint we
saw you as savages only. We discovered instead that you are unusual
creatures in your own limited ways. Ways which in time will not be so
RIKER: We're growing. Something about us compels us to learn,
Q: Yes, the human compulsion. And unfortunately for us, it is
a power which will grow stronger century after century, aeon after
RIKER: Aeons. Have you any idea how far we'll advance?
Perhaps in a future that you cannot yet conceive, even beyond us. So
you see, we must know more about this human condition. That's why
we've selected you, Riker, to become part of the Q, so that you can
bring to us this human need and hunger, that we may understand it.
So, although humans are quite limited currently, Q knows that perhaps humanity will eventually overtake the Q. So, one factor in choosing to be human may also have been to get back at the Q continuum, being part of a race that may overtake the one he was expelled from. By learning more about them, combined with his Q-knowledge, he could eventually become quite a powerful entity again!
Similarity of species: as @DavidS rightly points out, from the perspective of an extremely powerful species like the Q, the relative power of humans to other species is like comparing a snail and a slug! From our perspective, yes there's much more powerful species around, but from a Q's perspective, the difference could be quite negligible.