Feed in tariffs represent a great responsibility for the governments, and on the citizens taxes. These actors are now suffering too much the crisis. To my opinion, TGC can be more complicated and less secure for investors because they are more flexible. However, TGC represent now the most sustainable grant for green industries
This is very interesting question. Both mechanism can prompt the deployment of RES. However, TGC is a generation based, quantity-driven and has more uncertainty based on the market situation. The public authority set a predetermined amount of electricity which should be generated by RES.
On the other hand considering the investor's prospective, FIT which is a generation based prices-driven has a low risk. FIT has extensively prompt the deployment of RES specially in Europe.
I have attached two related papers "European renewable energy policy at crossroads—Focus on electricity support mechanisms" and "Analysis of Feed-in and Tradable Green Certificates (TGC) support mechanisms for renewable energy in Europe" hope you find them useful .