Seems that no matter wich one i choose, from certain point of view i choose wrong :D . My current setup have 2x Radeon HD 5770 so no matter wich one ill take it will be big leap from current :)
AMD and nVidia have their pros and cons :) You just have to weigh them yourself and choose how you want things done by your PC.
AMD - usually the better price per performance at the cost of power consumption
this case the R9 390 has alot of VRam paired with a good performing GPU. - Brute Force Approach
nVidia - More known for their Watt per Performance. Higher initial investment, uses better technology - Smart Approach
Bottom line - These cards perform very similar to each other with the correct configuration. Its just a matter of what pros and cons they come with that are acceptable to you.
If we were to talk about the religious holy war going on with AMD and nVidia about their principles like nVidia advertising their GTX970 comes with a full 4GB of VRam and not telling them that it will slow down if it uses more than 3.5GB and AMD's R&D Miracles by developing HBM and providing lower initial cost solutions to keep nVidia from monopolizing the GPU industry, then you will not arrive at an answer that will fit your needs (But hey, I heard they give you 72 Virigins afterwards!! havent really redeemed that coupon though.)
There is no Right or Wrong answer.
Just -- How do you want things done?? --