Lymphoma vs Leukemia - both are blood cancers and terms are sometimes interchanged by the public. But they have a little different presentation. Like rock said, lymphoma presents with the enlarged lymph nodes while the different forms of leukemia often do not.
Lymphoma is usually very responsive to chemo. And I agree that most dogs do not react to chemo as severely as people. We have such good anti nausea drugs that often that side effect can be successfully treated.
One of the hardest things about practicing veterinary medicine is dealing with the different levels of care that people expect and request. Some want an animal euthanized because of a broken toe nail (read sarcasm) while others will not see the suffering and futility of prolonged treatment in a hopeless situation. Couple that with having to suppress your own philosophy or feeling about what should be done versus what could be done.
I have been fortunate to have co-workers and a wife that offer a sounding board and our practice had a working environment that fostered that type of exchange. It's sometimes hard to keep things real.