Wow, at 17 surgery isn't that good for them either.

We just lost a cat to cancer about 2 years ago, and had to weigh the option of chemotherapy. We got a couple of opinions from the vets and they felt she'd probably only live a few weeks whether we treated it or not; they gave a mediocre probability of her surviving the operation. We brought her home so she could be in familiar surroundings till the end. Seemingly she was not in much pain and at least we didn't add to it. It's never a pleasant occasion, but I hope we did the right thing.
Good luck with your decision!