well,you could take either,but the turkish lira is their currency,not euros or british pounds,but some places will take them,but you will get a better deal with the lira.
I took sterling travellers cheques , which I exchanged to liras .
If you do take cheques , there are numerous places to exchange them - however shop around for somewhere where they dont charge a commision . The third place i went to didn't make a charge and their exchange rate was the same as everywhere else
I also took along my Nationwide debit / credit card .
There are only a few card issurers who dont make a charge for using their card abroad - Nationwide and The post office are two - so I would check to see if your card provider makes a charge .
Be aware though that you will be hit with a hefty charge if you use your credit card to widthdraw cash