Spay incontinence (fixed dog) versus potential unwanted litter and mammary cancer/ovarian cancer (intact dog). Neither spay incontinence nor mammary/ovarian cancer is all that common, but you are running a risk either way. And, if you run the risk of cancer, you are also running the risk of an unwanted litter. Personally, I'd rather run the risk of a dog that has a fixable piddling problem, then have a litter of unwanted pups and a dog with cancer.
That said, I've fixed every personal and foster dog (50+ over 15 years) that I've ever had, and I have yet to encounter a single incidence of spay incontinence. I have encountered one incidence of mammary cancer (it was fatal). And, I have regularly encountered unwanted litters.