[SixOfSpades]

I wouldn't go so far as to classify the Numenoreans as a "middle race" between Elves and Men, simply as a sort of "High Men" that enjoyed many of the advantages of Elvenkind, while still remaining mortal and bonded with the fate of Men.

As for "half elves being rare and not allowed to remain half elves," flip back to the section in The Fellowship of the Ring, just after leaving Moria. The Fellowship crosses a particular stream, and Legolas sings a song about the Elf-maid it was named after....the gist of it was that she left, journeying downstream toward Anduin and the sea....and then in Return of the King, Legolas meets the Prince of Dol Amroth (whose lands just happen to be situated near the mouth of the river Anduin), and salutes him, because it was clear that "the Prince had Elven blood in his veins." This line of Dol Amroth was also marriage-material for the Stewards of Gondor: Either Denethor or his father Ecthelion married their Princess--implying that they may have been trying to strengthen their dwindling lifespan by infusing some more Elvish heritage into the line.

"It's the divine ancestry of the Numenoreans and their descendants that sets them apart, not their Elvish roots." Since the two are rather inseparable, it's rather difficult to make any sort of claim on this point. One thing that can be said is that neither divine nor Elven blood can guarantee a person's greatness: Take a look at Elladan and Elrohir. As Elrond's sons, they were about 40 generations closer to their archmother (Melian) than Aragorn, and they've also got some very strong Noldor roots in their grandmother, Galadriel....and yet everywhere they go, they're riding Aragorn's shadow the whole way. And what about Arwen? All she did in the entire story was sit around and look pretty. Even Glorfindel was a bad-ass mofo compared to her.

As for the choice between Elf and Man, it would seem that the balance is fairly equal on both sides: If one were clearly better than the other, both Elrond and Elros would have chosen the same one. As for Elves envying Men over their ability to dream and the like, don't forget how much Men envied Elves their immortal lifespan, not needing to sleep, and of course thier ability to travel to Valinor. On the "green with envy" scale, I would say that the Men are definitely greener. Also recall the difficulty of Arwen's decision to bond herself to Aragorn's fate; she was sundering herself from family and friends, sure, but she was also giving up Eternity as well.