What do we do?

Mikey got Gus a "buddy" today (pictured.) This is very stressful to us. We love him so much, I just wish we could know what is the best decision. Planned Pethood will do the amputation for between $500- $600, but it really isn't about the money. I do have Care Credit http://www.carecredit.com/ , which is a "credit card" for veterinary service. They offer a no-interest time period, and is a great card to have during these trying times.
I am asking myself if I am wanting to go through with an amputation, chemo, possibly radiation for Gus, or is it us being selfish, and not wanting him to go? Everyone who has ever met Gus knows how special he is, and such a soulful dog. He is like no other I have ever met. Gus is not my "pet," he is my homeboy! We chill together, we play together, we used to hike together...