Which Generation 5 Pokémon game should I get?

as with all the pokemon games of each generation the main differences are in the version exclusives, so let’s begin with Black version, in Black you can find:

Weedle

Kakuna

Beedrill

Murkrow

Houndour

Houndoom

Shroomish

Breloom

Plusle

Honchcrow

Gothita

Gothorita

Gothitelle

Vullaby

Mandibuzz

Tornadus

and of course Reshiram

Now for White:

Caterpie

Metapod

Butterfree

Paras

Parasect

Misdreavus

Poochyena

Mightyena

Minun

Mismagius

Solosis

Dousion

Reuniclus

Rufflet

Braviary

Thundurus

and Zekrom

(Note that the legendaries are the opposite of what you’d expect from the game titles)

Now let’s move on to the sequel games in the generation Black 2 and White 2

First Black 2’s exclusives are:

Magmar

Magby

Spoink

Grumpig

Registeel

Latios

Buneary

Lopunny

Stunky

Skuntank

Gible

Gabite

Garchomp

Magmortar

Gothita

Gothorita

Gothitelle

Vullaby

Mandibuzz

Zekrom (and with it Black Kyurem)

And in White 2:

Electabuzz

Elekid

Skitty

Delcatty

Numel

Camerupt

Regice

Latias

Glameow

Purugly

Electivire

Solosis

Dousion

Reuniclus

Rufflet

Braviary

Reshram (and with it White Kyurem)

(Note that unlike the previous versions the legendaries are what you’d expect)

With all that out of the way there are very few differences between the 4 otherwise and like all of the games it’s mostly down to whatever you prefer

TL;DR
Just pick whichever one you like, they’re generally the same