Puzzle n°44 (**) - Which first prize ?

The somali of my friend Christine was elected « best of best » among the semi-long-haired cats at a cat show recently.
Christine was there, on the podium, together with the short-hair and long-hair winners.

There were three chests to be won, each one containing a prize. Just for fun, the organizer had decided to ask one of the winners to choose a chest, and Christine was the one. He explained that two of the chests contained a cat toy, and the third one a voucher for a magnificent cat-tree. However, he didn't mention any clue as to which one contained what... So Christine chose a chest. And the organizer opened one of the two others, after having told her it would be one with a little furry mouse-toy, which proved to be the case. He then asked Christine whether she preferred to stay on her choice or change for the last chest.

Of course, Christine only dreamt of one thing : win the cat-tree !
What would you have done in her place to have the best possible chances of winning the corresponding chest ?

... I'm giving up ...