Which is more effective in a 3.5" stud bay--"R13" or "R19" batt insulation?
R-13, will be. When R-19 is cramped in the same space, it is still R-13. R-19 will be a waste of money.

Use spray foam in the bays to achieve R-19.

Use your extra R-19 in the attic if possible, pull the paper off and layer it over the other insulation, if you have access.