The Heritage is stiffer than the Custom. I haven't been on the Heritage yet, but I'm gauging the flex by NS's chart given that I now know where their 6 flex rating feels like. The Heritage will actually be a good deal stiffer than the mid-flexing Custom. I'd say the Heritage will be a mid-stiff. If you want a board to focus more on freeriding rather than freestyle, go with the Heritage.
If you want a board to freestyle anywhere on the mountain, the SL or Proto will be a good option. I favor the Proto due to the Carbonium topsheet and a touch less dampening. It's not squirrely by any means, just feels lively. I also favor the Proto in that it has blunted tips and is lighter. Very good swing weight going on here.
And the last piece of the puzzle is the shape. The SL is a directional twin which has a slight setback stance. The Proto is a true twin which has a centered stance.
It's pretty neck and neck between the SL and Proto. My take is... they are both all-mountain freestyle. The SL is for more bias towards the all-mountain side of things. The Proto is for more bias towards the freestyle side of things.