>>57609626BW have a better plot, probably the least bad in the franchise (though obviously suffer as a game to replay as a result due to the constant dialogue)
BW2 have a shit story but a far better postgame (even if their postgame still kinda feels disposable compared to Platinum, HGSS, and Emerald).
Both represent what I think was the earliest point in the shift from pokemon games trying to be a sorta "world in a cartridge" with tons of content to be played basically forever, to "pocket sized JRPG", something you play through once, maybe do a couple of sidequests, and then wait for the next one.