Always opt for the non guaranteed rate. The guaranteed rate offered is terrible!
It will usually show you what the guaranteed rate is, and its always a fair bit below the current rates.
If you opt out, you will usually get the rate your own bank offers, so it will depend on what they give.
If you are using a UK card, you will probably be charged for withdrawing from a non UK machine, but I think you would be charged this regardless of with rate you chose.