I know this has probably been asked before but... I have a 3DS and was wondering what the differences are between B/W vs B/W 2? I completed the White version and was debating if it get Black 2.
I'd be glad to be of assistance, since I've played both pairs of games myself.
Basically, there's loads of differences between Black/White and Black 2/White 2. First, the Unova Pokedex. In B/W, only Unova Pokemon are obtainable prior to the National Pokedex, but B2/W2 has a mixture of both Unova Pokemon and several previous-gen Pokemon. Second, the storyline. B/W's story mainly focuses a lot on N and eventually ends with you catching Reshiram/Zekrom and then taking on both N and Ghetsis as the final battles as opposed to the usual Champion that would be the final boss in the main story. B2/W2's story is much different, and there's also new characters introduced (including new player characters).
Features-wise, it's also different. If you have the original Black and White, you can connect one of the games with Black 2/White 2 using the Memory Link (it does need a second DS though) and it unlocks cutscenes that won't appear without it, and also unlocks N's Pokemon available for capture. Seeing as you have a 3DS, you can also pay a little to download the Pokemon Dream Radar app which gives you some useful Pokemon that you can then transfer to Black 2/White 2, among them include the Therian Formes of Tornadus, Thundurus and Landorus.
My recommendation would be get the same color sequel that matches your original one, just because for example, Zekrom is in White and Reshiram is in White 2, but since you'd miss out on the Black version exclusives, you can get Black 2 if you want those Pokemon as well. Also, Black 2 and White 2 on their own have a lot of locations that differ in appearance and layout between versions.
If you're still confused, I'd highly advise reading
this post in the 5th Generation FAQ thread, and you'll soon learn that having both the original B/W and the sequels can enhance your Unova experience much better.