How do I decide when to put my cat down?

My 14 yr. old cat has a tumor on her back leg and up into her thigh. The vet says she'd suffer with radiation or chemo because of her age, plus we can't really afford those treatments. We have two other options--amputation if the tumor hasn't spread or euthanasia. She has arthritis in her hips, and she's a big cat. Amputation would cause her to put all her weight on the one hip to walk, and that's going to be painful for her. My vet is leaning towards euthanasia, and it looks like my only choice is to put her down. How do I decide when, though? She's eating, drinking, interacting with us, etc. She probably feels some discomfort in the swollen leg, but she's still walking around, jumping on the couch to sleep--as much as she jumped in the past, anyway, given the arthritis. Do I keep her around until she shows more signs of suffering, or what? This is so hard!