Ultimately, it's up to you. In my personal opinion, I think both games are just as fantastic as the other. It's really up to what you prefer more, enchanced graphics/new pokemon, or that nostalgia factor (with some still pretty good graphics!). Storyline-wise, I'd opt for BW2, (not BW). The original Black and White proved quite boring, but that may just me. I found the storylinbe incredibly short - especially the post gameplay, which didn't equate to the tons of replay value we received with HGSS.
All in all, it's your decision. But in this case, I'd go with Platinum, and then jump up to Black 2/White 2; start with the classics, y'know.