Up to you, really - certainly HGSS are decent games overall and worth a go, but maybe consider getting one of them after they have a price drop which should happen sooner or later. They're not 'necessary' per say unless if you want to complete your pokedex easier, but then no game can be really described as 'necessary'. =p
Guess that's pretty much the advice that'll do, as people will suggest different things - if you don't care about saving money get them now, otherwise wait a bit with one. (Also as with which is better out of B/W, depends which legendary and version exclusive Pokemon you prefer, as well as if you prefer one game-specific area over the other - check out bulba or serebii or the such for more on that).
Closed.