"Select your prize" promotions on slots

From time to time, I get some sort of promotion in the mail from a casino that goes like this:

Insert your card in your slot or VP machine and choose one of three "surprise gifts" which range from $XX to $XXXX of free play on the "rewards window." (This particular casino has a screen on the machine that shows promotions, point count for their players club, etc. And other casinos have a screen at a players club desk for similar promotions._

My question: are these "prizes" predetermined by "casino marketing" or are they really "random" as the players clubs tell me they are? So, does it make a difference if I choose among the options (there may be a choice of 1, 2 or 3) or are all three "buttons" the same?

Anyone with inside info on these?