ha. 764s are all PMCO and the 763s have recently started undergoing refreshes that are more in line with the PMCO interiors at least for the BF cabins, but are still mostly PMUA IPTE (International Premium Travel Experience).
what is "best" will have do with your own physique. the 764s are all converted to the 39 BF seats, apart from the ones used for the Hawaii flights, which still use the legacy barcalounger seats. i think all of the 763 ghetto birds are now offline, so there are now 3 class IPTE PMUA furnished ones, and some recent refreshes that feature the [B/E Aerospace?] seats like the PMCO seats found in the 772 and 752 and now the 764.
apart from the seatmaps and specs shown on UA.com, you can look at this site which tracks the interiors of the entire fleet.. i actually now have it book marked.
https://sites.google.com/site/unitedfleetsite/mainline-fleet-tracking
from the PMUA v. PMCO Biz lieflat seat debate (not talking about AVOD), the PMCO seats tends to be a bit wider and has more personal storage space, where as the PMUA tends to be "longer." taller pax aren't too fond of the PMCO styled seats and from the feedback i see on FT, it seems there is preference for IPTE. i recently flew a refreshed 764 and the IPTE 763 and liked both. for me, i'm 5'10" and of a slim/moderate build so both are fine for me w/ respect to seat length. they layflat and i can fit in both without feeling squeezed. both have armrests that drop when you are you have an aisle.
another nice thing about the IPTE 763 is that you don't have the middle block of 4 seats like on the 772 and 747.
for the 764 and 763 refreshes has a single aisle for the "D" seats so you have aisle on both sides which is nice when you are traveling solo.