Jen,
We just lost our dear golden retreiver 2 weeks ago to cancer. Last October we moved to Luxembourg, EU so we had to give him away to our neighboors. We could have taken him as there are no restrictions here but our lifestyle (lot's of travelling) would not have been fair to him. He was 10 at the time. 3 weeks ago we got a call from his "new family" that he was having a lot of problems breathing so they took him to the vet. It was the same diagnostic as you. He had a cancerous tumor to his spleen that spread to his heart and liver. He was retaining a lot of fluid which made him uncomfortable but not in pain. The tumor was too big to treat. We (the new family and us) decided that as long as he was not suffering they would keep him as comfortable as possible. We just happened to be in the US at that time. My daughter and I went to visit him. That day he was not doing well, his breathing was very labored and he was confused and tired. It got so bad that they took him to the vet at 3am in the morning.
He then had a lot of fluid in his abdomen that his body could no longer absorb. Draining him would have been futile as it would come back. By then he was in pain so they (we had discussed it prior) decided it was best to put him down.
I guess what I'm saying is that if you feel the vet can keep your pet comfortable and you can enjoy more time with him, go ahead and enjoy. But if your dog will be going in and out of pain and discomfort your thoughts should be about him.
Good luck and I'll keep you and your "pouch" in my prayers. :-(