Oil has an operating range. Too hot or too cold and things don't work the way they're supposed to.
For a year round daily driver, you would want the add on cooler before the radiator cooler. That will provide good stability of the temps even when it's really cold. It will shed most of it's heat before hitting the radiator, and the radiator will provide additional cooler or add some heat if needed.
For a summertime fair weather fun car, probably put the coooler after the radiator, so you can shed as much heat as possible.