I don't understand what you mean by "detected cancer" and "symptomatic cancer". Eg if a cancer is already detected then by definition it is impossible to screen for it.
I see, it should have been written "screening-detected" to be as clear as possible.
Yes, the latter is more likely to be comparable over time but it will still have issues with changes in how likely someone is to report it to their doctor, etc.
vs
(Symptomatic detected) Hey, what is this lump here? Why am I so tired? etc...