When I was a kid we found out that our dog had a large tumor. The vet told my parents that they could operate, but it would be very expensive and the tumor would likely come back, and so they recommended putting her down as she was in a lot of pain from the tumor. My parents came home and told us kids this news before deciding to schedule the appointment for the next week. A few days later when we got up in the morning, she was dead. I'm not sure if you have kids or not, but the point of my story is if you do, make sure that they are aware of what is going on, no matter what you decide as it sounds like your cat might not have much longer either way. Losing my dog was hard, but it would have been much harder if I was completely blindsided by it. It's a tough situation to be in, and you have my sympathy.