I always take cash and my cards as a back up (i always spend more than i plan to!).
You can take sterling or euros and there's not alot in it cost wise. The rate is normally slightly better in Rhodes but they charge 2% commission so if you can get a good rate here comm free it's pretty similar.

We always take a couple of hundred pounds worth of euros when travelling anywhere in Greece and then use our Nationwide debit card in ATM's. Nationwide give a decent exchange rate and make no charges for foreign transactions. If you don't have a flexaccount with Nationwide, open one! It really is the cheapest way of getting by abroad.

I have no connection with Nationwide apart from being a satisfied customer.