For most stuff the 6082T6 Flat bar is ok but it's NOT flat so if you need perfectly flat then be prepared to surface both faces and obviously lose some thickness.

What I do is buy the 6082T6 for none critical parts like gantry sides, motor mounts etc then buy the Ecocast for things like Z axis and bearing mounting plates etc.
Doing it this way saves me a load of time because I don't have to machine any critical areas like bearing mounts flat and basicly can bolt straight to surface.

The 6082T6 flat bar in wide pieces it's seldom flat and has a slight cup to it and in critical areas like bearing mounts then chances are you will get some binding in the bearings. . You can get round this and keep some thickness by machining just the bearing mounting areas, which is what I used to do but it's time consuming. The ecocast is much quicker and more accurate better looking.

I recently got some of the 5083 series cut plate from them and that was resonably flat and a bit cheaper than ecocast, it cuts ok as well. Ecocast would still be my prefered choice for the Z axis thou.!!

Edit: Ecocast is no easier to machine it's exactly the same to work with as 6082T6 it's just precision ground flat.!!