Gokdeniz is a great player too, he had some problems a few years back regarding some issues with a betting company (betting on his own matches) and was banned from footy for a while, but overall he's a good, honest and helpful player. He's been good for the Turkish national team this year and if you were to look at the match highlights of the Turkey - Greece game a few months back, you would have seen his rather amazing goal that he scored 2-3 mins after coming onto the pitch.
About Tuncay, the only important thing is that Tuncay should be played as an AMC or a Forward... he is good on the wing but its not his main position. I don't know what your manager intends to do with him so I can't comment, but Gokdeniz is a good winger and can play R or L.
Boro will have quite an interest from Turkey next season, I myself along with many Fenerbahce fans will watch our captain play in the EPL and will root for you guys

smart transfer I'd say. I hope he does not dissapoint due to adaptational problems.