If you have a better cube that you aren't using specifically for CFOP practice currently, I'd recommend using your main/best 3x3 for your practice. Even if you think the rougher turning of the Rubik's brand will help your finger strength, the quality disparity between both cubes when you return to your better cube will be more dramatic than the negligible strength benefits of using a slower cube. If you're concerned about deterioration of the better cube from overuse, then you can lube it periodically with speedcubing lube as it begins to feel dry or scratchy.
If your Rubik's 2.0 is your only/best cube, then I'd recommend buying a better speedcube from an online retailer (such as thecubicle.us) rather than buying a new core for the Rubik's 2.0.