30 kg luggage allowance for business class as is written on their web page.
http://cmss.atlasjet.com/tr/kampanyalar/BusinessClass

Most airlines and planes flying local in Turkey do not have special business seats.

You usually get the middle seat empty and better food and beverages on business, but it is not worth it, unless there are no vacancies in economy because the flights are usually around one hour and you get a soft drink, water coffee.tea and a sandwich or salad and cake (on THY) in any case.

The larger THY planes will have the business seats, but you need to double check and make sure that your schedule has them.

Having just done some research (and booked my flight via AtlasJet), I can tell you that THY is MUCH more expensive. For me, the legroom in business class wasn't that important (it's only a 1 hour flight), but the extra luggage allowance was. Also, AtlasJet had good customer reviews on airlinequality.com

We've flown Istanbul to Izmir on Atlas Jet twice - it was a smooth flight. Takes about 1 hr so is there really a need for business class on a short flight? The airport in Izmir is modern, but very small. There's a bus from the airport to city center/Izmir that's much cheaper than a taxi. ENJOY!

thanks all.
debs-- can you tell me about your experience with luggage? on their web site, it says 20kg for economy... but it seems like that is the limit for carry-on only. ??? it's a little unclear. can we plan to check a bag of 20kg and then carry on a small bag?? it's important, because this will be at the end of the trip. when we fly home to the u.s. we will have a very large baggage limit and hope we don't have to pay a bunch extra on the flight izmir to istanbul.

8 kg and 25x45x56 cm bag is allowed in the cabin in addition to your 20 kg checked luggage in the economy class

thanks much, senozkadir!