tumors

Hi, I took Kelly, my 11 year-old labrador x staffy to the vet as her belly seemed to be dropping and her spine was sticking out more. Her incontinence seemed to be getting worse too. The vet did an ultrasound and said he could see several black spots on her spleen and liver, which indicated tumors. He suggested putting her to sleep earlier rather than later as it is only a matter of time before one of the tumors could burst. I am not sure what to do. Kelly still has a huge appetite and is wagging her tail. She also enjoys going for walks/swims, though doesn't last as long as she used to. Her legs seem a little weak today. Should I take her to get a more thorough diagnosis (not just an ultrasound)? Many of her symptoms seem to point to Cushing's disease, but I guess I am just being hopeful and there is no reason why the vet would say she has cancer if he wasn't sure.

If I do just accept that she has incurable cancer and it is just a matter of time, I don't know how long to wait. I don't want her to be uncomfortable or get to the point where a tumour bursts, but I find it extremely difficult to think about putting her to sleep when she still seems fairly happy and energetic.

I'd appreciate any advice.
Thank you.