I am feeling the same way. Have Y somewhere. But the story aspect in BW sounds interesting.

So it comes down to this. Which one has the most pokemon. Also which one has the best final dungeon or endgame if you want to call it that.

Do you mean the most NEW Pokémon or the most in general ? The most New Pokémon are in B/W and it's sequels since it introduced 156 Pokémon while in X/Y you can find about 500 Pokémon but only 72 are new (but among the 500 there are also some of the 156 from B/W). X/Y are really easy while Imho B/W and it's squels are more challenging and have some nice labyrinths that'll take you some time to figure out...

I guess when it came to pokemon composition I would want most of the original 150 (dont care if Polygory and a few other of the misses in R/B are not there)

then X/Y is the choice for you, there are almost all of them,  you can even pick the starter! In B/W I believe there's only Ditto and in  B2/W2 there are more  but not as many as in X/Y

that is great because I own that already. Time to dig it out.