Since you are traction limited I'd think the power difference matters less than the shape of the torque curve. And a roller cam would give you more control over the shape of the torque curve since you can get a lot more creative with the lobe shape. So if it was me, I'd start with a roller cam and never even bother using a flat tappet. I'm not saying it is going to be easy to find the best torque curve to match your chassis and your driving style, but with a roller cam it will be a lot easier to make adjustments.