opinions on 970 or 980 4mb cards

This is only for Prepar3d so what would you get or what have you got.

I run Orbx vector ,global and OpenLC plus Rex textures and the Aerosoft A318/19.very heavy hit with MytrafficX 3d and UK2000 airports.

I'm using a 770GTX with 2mb and it's not quite up to it.so the question is...

what card and will the extra 2mb of onboard memory help .is the much of a difference between the two??

 thanks for any thoughts

     steve