I am very sorry that Benny has been diagnosed with Lymphoma. My heart truly goes out to you both. First of all know there is no wrong answer here, whichever you chose for your Benny he will be kept as happy and as comfortable as possible for as long as possible.
The doses of chemotherapy used in dogs are smaller then those used in humans, therefore the side effects tend to be lowered. Dogs do not generally become as ill as a human undergoing chemotherapy does.
Benny has lived a very long and I am sure happy life with you. A year can be a gift on top of that life, as can a further 2 months.
I know the decision to let go of one of our pets is incredibly challenging as you move forward with either treatment keep that in mind that as veterinarians we strive for a quality of life for the pets we see. If you believe that Benny is still happy and has a good quality of life, then there isn't a reason to let go of him yet. It can be difficult to be objective when it comes to our own pets, so discussing with other people in Benny's life as well as using a quality of life index can be helpful. A quality of life index, like the one at the link below, asks you questions in order to obtain a quality of life score. It is advised that you take the test three times on three separate days. It is also helpful to have others who know Benny well answer the questions. The aim is for a score above 35, however, it is only a guide and ultimately you and Benny's veterinarian understand great situation better then anyone else.
http://www.aplb.org/resources/quality-of-life_scale.phpI wish you the warmest regards in this decision. I can't advise what is the right choice for you. But if it were my dog and finances allowed I would want to try for the extra year given by chemotherapy. Mainly because I believe you would have more healthy time with him in this scenario, whereas with using on prednisolone he will continue to deteriorate from this point on. But as I mentioned there is no wrong answer. You are doing him well in either situation. In either situation you will treasure the time you have left together.
I would be will be very difficult when the time does come, but try to focus on the love you two shared, and the memories you made to create a beautiful lif for dear Benny