You have the specs for either solution.
You will win FPS benchmarks with cf R9-280 cards.
The price you pay will be the usual dual gpu issues such as stuttering and screen tearing.
You will also have a noisier and hotter rig.
Unless you are gaming on triple monitors, or a 4k monitor, I would pick the GTX780ti which will do the job very well.
Yes, you will have to market the R9-280X.
Find your parts in used condition on ebay.
Then filter on completed auctions,
In green you will see what the part actually sold for.