Three-way SLI GTX 970 + 750Ti for Physx or Quad-way Crossfire R9 390? 3D Rendering and Gaming

In terms of overall gaming performance,
the graphical capabilities of the Nvidia
GeForce GTX 970 4GB are marginally
better than the AMD Radeon R9 390.
The GTX 970 has a 50 MHz higher core
clock speed than the Radeon R9 390 , but
the Radeon R9 390 has 56 more Texture
Mapping Units than the GTX 970 . As a
result, the Radeon R9 390 exhibits a 50.8
GTexel/s better Texture Fill Rate than
the GTX 970 . This still holds weight but
shader performance is generally more
relevant, particularly since both of
these GPUs support at least DirectX 10.
The GTX 970 has a 50 MHz higher core
clock speed than the Radeon R9 390 , but
the Radeon R9 390 has 8 more Render
Output Units than the GTX 970. As a
result, the Radeon R9 390 exhibits a 5.2
GPixel/s better Pixel Fill Rate than the
GTX 970 . However, both GPUs support
DirectX 9 or above, and pixeling
performance is only really relevant
when comparing older cards.
The Radeon R9 390 was released less
than a year after the GTX 970 , and so
they are likely to have similar driver
support for optimizing performance
when running the latest games.
Both GPUs exhibit very powerful
performance, so it probably isn't worth
upgrading from one to the other, as
both are capable of running even the
most demanding games at the highest
settings.
The Radeon R9 390 has 4096 MB more
video memory than the GTX 970 , so is
likely to be much better at displaying
game textures at higher resolutions. This
is supported by the fact that the
Radeon R9 390 also has superior memory
performance overall.
The Radeon R9 390 has 159.6 GB/sec
greater memory bandwidth than the
GTX 970 , which means that the memory
performance of the Radeon R9 390 is
massively better than the GTX 970.
The GeForce GTX 970 4GB has 1664
Shader Processing Units and the Radeon
R9 390 has 2560. However, the actual
shader performance of the GTX 970 is
2646 and the actual shader performance
of the Radeon R9 390 is 2176. The GTX
970 having 470 better shader
performance is not particularly notable,
as altogether the Radeon R9 390
performs better when taking into
account other relevant data.
The GeForce GTX 970 4GB requires 145
Watts to run and the Radeon R9 390
requires 275 Watts. We would recommend
a PSU with at least 500 Watts for the
GTX 970 and a PSU with at least 600
Watts for the Radeon R9 390 . The
Radeon R9 390 requires 130 Watts more
than the GTX 970 to run. The
difference is significant enough that
the Radeon R9 390 may have an adverse
affect on your yearly electricity bills in
comparison to the GTX 970.

Many games now start using nvidia gameworks and physx to add more detail in graphics.So having a nvidia card is worthy.

As the both cards are of same potential so having a gtx970 sli is best.It can beat a single gtx 980 in the same range of price.

No i meant to save up some more then make your purchase. Also take into account 970's only have 4gb of memory (3.5gb high speed).