As someone going through a similar dilemma with my own bitch who has Lymphoma, I totally sympathize. I think this is a very individual decision, there is no overall right answer, just the right answer for your circumstances. I don't know if that helps.
Just one thought. It's unusual for a 2 year old dog to have Lymphoma, and the fact that your other dog was also diagnosed with Lymphoma in the past year makes it more suspicious. Could there possibly be a link, i.e. some carcinogen around your property, lawn chemicals used by a neighbor, etc.? Just something to think about, not only for the dogs, but also for your family's health.