Actually, the higher magnification works better, as far as it's focus length is not too short to burn the lens when tinder ignites. But JCWohlschlag is absolutely correct that the lens diameter (surface area) is a more significant factor.

I'd recommend something not higher than 5x (2 inch focus). The physics behind this is simple. The shorter the focus distance (higher magnification) - the smaller the spot where the energy from the Sun is concentrated (and so higher the temperature).