Well, BM vs. MM in PvP will continue to be a hot debate until all of the wrinkles are ironed out. I honestly don't expect any kind of realisitic consensus HERE until perhaps after Christmas, and even then, it may drag out a while. Not only is the theorycraft changing by the day at this point, but the impacts of the talents, the combinations of pet talent trees, etc. are all factoring in for many siginificant possible changes in how a hunter gets played in PvP.
I am already seeing champions of each approach but given the time needed to TRY various approaches to see what works and what doesn't, well, I certainly wouldn't be willing to make any sweeping statement of support to BM or MM or SV for PvP or any other use. We can't even nail down a realistic autoshot count on PTR/Beta by the looks of it. So the final 'release' of LK will only be step 1 in the process of vetting the theorycraft. Rinse, wash and repeat with each 'fix' and 'change' pass that LK will go through...
Rhino's look good for PvP (knockback/down is a great interrupt/stun effect) but it is just one effect of many possible pet combinations. I wouldn't doubt if we don't see Hunter/Pet builds customized for a given 'team' role. You'll probably see this more in 5v5 than down at 3 or 2v2. Rhinos are also good vision blockers, nasty for a player with a Rhino attacking them, and a big red Rhino can swallow a whole screen by the looks of it, then again, so can a 'saur. I think BM 51 with a Rhino will be a common pick for many players, PvP or otherwise. Will Raid bosses or Elites be subject to knockback? These questions and more to be answered...