First off I would stay away from used components. No matter what, something could always go wrong, and then there is no chance of warranty of RMA.
With that being said, I would most likely go with the GTX 780 TI as it is just a bit better than the R9 390 8GB with the exception of the graphics department, as seen here: http://gpuboss.com/gpus/Radeon-R9-390-vs-GeForce-GTX-780-Ti.
Even then, that is just a guide. You also want to think about what your PSU can handle.