so what is better to buy for final cut pro a reference AMD or Nvidia?
It depends is the only answer. What's the best car for you depends on lots of factors, e.g. cost, carrying capacity, speed, acceleration, fuel economy. Graphic cards are the same.
AMD cards are very good for FCPX as FCPX is optimised for AMD and the cards are fast. They also tend to work out of the box, at least for the 280X range (lets ignore XFX cards). nVidia cards are getting better but at the moment if you wanted a dedicated FCPX machine, AMD cards are probably the best option but the difference is now slight over a few years ago.
However nVidia cards tend to need drivers whenever a new OS X build comes out, some people don't like having to do the drivers update. Also if you use other software, nVidia might be a better choice and you take a performance hit on FCPX. This performance hit might be negligible or a lot depending on your workflow.
You need to look at what you need, most people don't need the very last % of power from FCPX and so an nVidia card may be best, some people like the ease of use of the AMD cards and the fact you can run multiple AMD cards easily.
You need to read and work out what you want to do, its never as simple as what's the best. Whats the best car, an Audi A6 or a Mercedes C-Class?
i will just get the Gigabyte 460 4GB i am just building this rig for Final Cut pro..