Turkey and Greece are miles apart economically.
Turkey's is one of the top 20 economies in the world and is better than more than 20 existing EU member states. Its GDP is about 4 times that of Greece.
The main reason for barring Turkey's EU membership (other than the Greeks object to it) are more political than economic. France who share most of the power with Germany are against it purely because Turkey is a Muslim country.
If Turkey had been admitted instead of Greece, the EU and the €uro woyld not be in its present mess.