According to people on Tripadvisor and across the Internet, the consensus is to take Turkish Lira rather than Euros. One user on Tripadvisor says to take some Euros with you, or use up what you have left from a previous holiday, and then take Pounds Sterling out with you to exchange into Lira. Apparently, the exchange rate is better in Turkey than it is in the UK.
What if I'm not from the UK?
It seems like it would be best to just take Turkish Lira with you if you're not from the UK, as then you will have the currency ready to spend when you get there. Turkey accepts three currencies - Lira, Euro and Pounds Sterling, so really it's not going to matter what you take out of those three. However, many tourists have agreed that you get more for your money on the Dolmus if you take Lira.
I hope I've helped you make a decision. To get you in the holiday mood, why not watch this video on holidays to Turkey? Hope you have a great time!