Who "decides" which opening to use?

yes your are right I did mean Pawn structures and not chains, thanks for pointing it out....

no he is not, lol, clowns

NM

The opening is usually decided by a culmination of moves between both sides.  Either side can limit the amount of variations that the other side can choose if they want to, but there isn't really any way to limit it completely.  My opening repertoire right now severely limits the options of my opponent (as white, I play Bird's opening and as black I play the Scandinavian against 1.e4 and the Dutch against 1.d4), but even trying as hard as you can, there are still other ways people can take the game (for example, the blackmar-deimer gambit against the Scandinavian, or From's gambit against the Bird).

I think in general, white determines the opening. In my opinion, this is a huge part of the alleged "50 rating advantage" that white has.

It's true that both players make the opening (and this is a really cool part of chess) but white's decision to play a c4, d4, or e4 game is a pretty huge one that black has pretty much no say in. The pawn structures and patterns for each are very different and players who are defending against opening moves they don't play can have a lot of trouble if they are novices.

Black's decision to play the Sicillian, for example, doesn't really mean as much as white's decision on how to attack it. A lot of sicillian lines transpose for black and black doesn't always have that many things he can do, but white can decide whether he wants the game to go slow or whether he wants a fast f4 + queenside castling game. If white wants a slow positional game or a sharp tactical game, how much can black really do to stop it? It seems like white always has the first choice as to how he wants the game to go.

But it could all just be an illusion.

Aidan.... with black try. accelerated dragon, dragon, Dutch, Leningrad. Grundfield.... with white, the first couple of moves of the Reti. the Nimzo-larson and the Birds... all work well together....