Hi Michelle, welcome! Hope you don't mind I moved your post here but this is a good place to talk about pre-surgery questions like this.

So my question would be, what does your vet recommend? Did you see an oncologist to get a workup on the cancer? If there is spread usually a CT scan will show but where it spreads depends on the type of cancer as far as I know, and that's what determines where the CT scan will be done. For example, osteosarcoma usually metastasizes to the lungs so x-rays and often now CT scans are done to check for mets in that location. The question is, what would you do with that information if the diagnostics did show spread? Would that change the course of the treatment you decide to do?

Soft tissue sarcomas usually re-grow, it's difficult to get the entire cancer so that's why amputation is more curative (usually).  

The good news is that a young Border Collie has every chance to bounce back and be just as bouncy as ever. They're pretty resilient! 

You've definitely done all the homework and I can see why you would be reluctant to go forward with the amp if there are mets. Many people face this decision. A CT scan can certainly pick up "micro metastasis" better than an x ray so if that's in your budget it's one way you can decide with more certainty.