I was faced with that choice when my cat William was hit by a car 3 years ago. His front leg was shattered but I was told there was a 50% chance they could save it and they'd like to give it a go. I decided to let them try and save it even though it would have meant a long period of immobilisation and recovery before the leg could be used. In the end it wasn't a success and the leg was amputated a few days later anyway (and he is still fine!) but I still don't regret giving him that chance. You have to make the decision you think is best for you - I'm sure you will and I wish you all the best.
Cilla and William