Istanbul is great for nightlife around Taksim, and sightseeing on the european side, but it is hot, very overcrowded, and you will need a guide.
Bodrum & Gumbet are both excellent beach venues and do have great nightlife, full of english and turkish muff.
Marmaris is a more upmarket harbour resort and full of germans, great restaurants and bars.
Alanya is excellent for beach, nightlife and fantastic for sightseeing as it is towards the east. By far the best venue for visiting sights such as Cappadoccia, Pummekala, Troy and Batman which is a PKK stronghold, but well worth a visit. Alanya is bad for mosquitos mind.
Kusudasi (port), Fetiye and Olu Deniz are very lazy beach only resorts.
Any further questions or advice you need mate PM me.