Which door?

Mr Gibson from Maths department has asked me to challenge you all together with this little brain teaser. Try to solve it together - more heads know more than one!

You are a contestant on a TV game show. Before you are three closed doors. One of them hides a customised, freely-installed waterwise system for your home worth $15000 (which you want to win); the other two hide annual subscriptions to Advanced Hydrology magazine ($35, which you do not want to win). Excuse these lame choices ...

You get to pick one of the doors, and you will win what is behind it.

However, the way the game works is that the door you pick does not get opened immediately. Instead, the host will open one of the other doors to reveal a magazine subscription. He will then give you a chance to change your mind: you can switch and pick the other closed door instead, or stay with your original choice.

Which of these two strategies gives you the better chance of winning the water system you want?

Which strategy is better: to stick with your original choice, or to switch?