Oh dear, tough one. Here's what i would be considering.
How is the staffy at dealing with the vet, having walks and so on limited? Either way it sounds like there would be a strong risk of incontinence so if it was me I would discount that as a factor. Is it DEFINITE that its a cancer?
If its IV chemo then the dog has to be happy to be handled, to accept multiple insertions of an IV line, because I think that the chemo is more than one dose, and to stay quiet at the vets while the IV line is in. I also think that there is no guarantee that this will prevent the spread of certain cancers. I understand from dog owners who know, that dogs tend to tolerate chemo better than people in terms of sickness loss of appetite and so on, but some dogs still feel rough for a day or so after each dose.
I have got a friend who rescues greyhounds. They have a high incidence of bone cancer, the treatment for which is amputation and chemo. She knows that some of her dogs could stand the stress of the treatment as I have outlined above, and that for others, the stress would be worse than early PTS.
I hope this helps even a little.....