This is a tough one. You just have to hope you'll know when the time is right and have the strength to do it. Keeping a loved animal in pain seems much more cruel than ending their suffering.
There is no doubt about it being a hard choice to decide. I have to agree with Gunslingergirl though. I went through this once with a l black lab that was 17 years old and it was hard. Watching her struggle to get up in order to go outside and do her stuff got to me. She was on meds to help her cope but seeing her suffer was really hard to watch. I had to concider how good she was for us and then decide what was best for her as winter months were moving in. It was a sad day to make that choice but it was made and she was laid to rest. When I think about it, it is still sad to have done it that way but, it was the best I could have done for her IMO. Good luck with your choice.