If you take a general look at the fleets of the exsisting fleets of the charter airlines in Turkey you'll see there is an uprising trend around to switch A-320 family.Ofcourse the target market of the airline you're consulting is important for the aircraft chooice.Will they only carry tourists and Turkish workers from Europe or are they planning to hop into domestic business too which had an giant jumpstart since the last year.
As I can recall only two operators were using 757s and one of them was complaing about whatever they do they cant reach the economy of 737-800 and were leasing our planes instead of 757 and they still could make profit(and so did my ex-company)
Both Turkish airlines and Onurair are the biggest A320 family operators,they have technical backround.They can provide flight training also.I guess they should send pilots abroad to train pilots and techicians(which wil cost a lot) for 757 ,instead of using the capabilities of the existing Turkish carriers which will be a cheaper option.
I am assuming your clients will not invest millions of dollars in advance to stock spare parts(like most of the charters do,they buy it when they need but that costs delays even leasing aircraft from another airline to save the day) 757 will take alot longer than A320 which you can quickly solve the problem from THY or OnurAir stocks on the opposite side of the ramp.For 757 you'll have wait until it will be send to you by the first available plane(which you have to pay for it too)
On the otherhand 757 has very powerfull engines which will be very useful for operating to shortest and less used airfields.(which has cheaper landing fees)Especially when carrying Turkish workers in and out from Turkey they carry alot of heavy luggages(present for the family and Turkish food on the way back as you can guess
)757 will be less limited(or leave less baggages behind) which will cost you extra money to shipped behind.
Althoug I dont know what category both these aircraft fell into but overflying and landing fees are also vary from type to type too.So you should consider this also.
There are a lot of variables you have to work very hard to find out.This is a very big decision you all have to make.Think a lot before hand,so we(pilots) dont have to think about how we can be paid if the company is not profitting.
Good Luck. WING