Apologies if differences between 777s have been discussed in detail before; I searched and didn't see this question answered.

For a long-haul flight to NRT this fall (mid-October) my options are either UA7 IAH-NRT or UA803 IAD-NRT. UA7 appears to always be a PMCO 777 while UA803 frequently is listed as a new configuration 3-cabin 777 (I understand that it is close to a toss up as to whether I would actually get an "old" or "new" 3-cabin 777 ex-IAD).

If I will be in Y (and likely not Y+) for the long flight, what are the differences between the PMCO 777 and the 3-cabin 777s? Is one significantly better than the other in Y? Is the "new" Y on the reconfigured 3-cabin 777 worth the gamble over the near-certain PMCO 777?

Any other reason to chose one of these flights over the other? I have UC membership, so the LH lounge at IAD is about the only difference I can see (though I've heard that the Concourse E club in IAH is quite nice).

Thanks!