Koodo's international travel add-ons should work in Greece without difficulty. If you need a large data bundle, a local SIM may be the better choice.
Edit: If you are a prepaid customer, Koodo offers only US travel options.
If you are going to talk a lot and use a lot of data go with local SIM. If you have to be in touch on your own phone number while you're travelling and maybe use it for the sake of emergencies choose cheapest add-on from Koodo.