10.5 yr old Airedale has a rectal tumor. Chemo or not?

My 10 1/2 yr old female Airedale was diagnosed with a rectal tumor yesterday. It's the size of a quarter and is very close to her anus. She is constantly licking and seems like she's trying to poop out the tumor. The vet said that they can operate by doing a rectal pull, where they pull out the tumor through her anus and cut it out that way. There is a risk of permanent incontinence afterwards, but they say that the chances are small. There's a 50/50 chance of getting all the cancer cells and paired with chemo, there's a good chance that she'll fully recover (if there aren't any complications). The tumor is locally aggressive (meaning that it most likely won't spread to other organs) but will get bigger and bigger until she won't be able to poop anymore. The average lifespan of an Airedale is 11-12yrs, so the prognosis if we do surgery and chemo is probably a year (and who knows if she'll need a second round of chemo treatments). If we just put her on pain meds, prognosis is 4-6 months. I love her so much, but I don't want to put her through surgery AND chemo (we're looking at 4 months of chemo) and then go through a period of not-so-healthy times followed by maybe 2 or 3 months of semi-ok health followed by a fast decline of health for a month or two. They want to schedule her for surgery at the end of this week, but today she was out playing with our two other dogs (we have two of her pups from her litters) and we went for a nice walk around the block. I don't want to take these happy times away from her.... but I also don't want her to be in pain.... but I also don't want to cause so much physical stress on her just because I want to have her around for a few extra months longer. What do I do?? The surgeon phoned us today and made it seem that if we don't do surgery, we'd be doing her a great injustice by giving her 4-6 months of pain leading up to the tragedy of not pooping...... he said if it was his dog, he'd do the surgery in heartbeat (but he also doesn't have to shell out $5000+ for surgery) but our oncologist says that whatever decision we make will be the right one. I just don't want to be always asking, "what if". Help!!