ORing diodes is sufficient.
Are you expecting the Arduino to source a lot of 5V current to a shield, LEDs, anything?
If so, an external regular to bring the 9/12 down to 7.5V before feeding into the barrel jack connector, or 6V to feed into Vin, would be a good idea.
how can I brig down both bot voltages?
A this moment I'm planning to interface a temp sensor(LM35) and a RN42 bluetooth module but maybe I'll add a gsm/gps + sd card, but I'm not sure if I can interface everything at the same time.